XBRL and Inline XBRL Tagging
17 CFR 232.405 (Rule 405 of Regulation S-T)
XBRL (eXtensible Business Reporting Language) is a structured data standard that the SEC requires for financial statement data in periodic reports. Since the completion of the Inline XBRL phase-in (2019-2021), all operating companies must embed XBRL tags directly within their HTML filings using Inline XBRL (iXBRL) format, rather than attaching separate XBRL instance documents.
The requirement is codified in Rule 405 of Regulation S-T (17 CFR 232.405). Filers must tag the face of their financial statements, notes, schedules, and cover pages using the standard US GAAP taxonomy maintained by the FASB, with custom extensions permitted where no appropriate standard element exists.
US GAAP Taxonomy
The US GAAP Financial Reporting Taxonomy is the standard set of XBRL elements used for tagging SEC financial reports. Key facts:
- Maintained by the Financial Accounting Standards Board (FASB) and updated annually
- Contains approximately 18,000+ elements organized into presentation, calculation, definition, and label linkbases
- Elements are categorized as monetary, per-share, shares, percentage, string (text block), or other data types
- Dimensional modeling uses axes (dimensions) and members to represent disaggregated data (e.g., segment reporting, product lines, geographic areas)
- Each taxonomy release is aligned with the latest FASB Accounting Standards Codification (ASC) updates
- Filers must use the most recent taxonomy available at the time of filing
Custom Extensions
When no standard taxonomy element adequately represents a concept, filers may create custom extension elements. SEC guidance on custom extensions:
- Custom elements should only be created when no existing standard element has the same or substantially similar meaning
- Each custom element must include a label, definition, and reference to the relevant accounting standard or regulation
- Custom elements must be anchored to the closest standard element in the taxonomy (required since 2025 taxonomy updates)
- Excessive use of custom extensions when standard elements are available is a common SEC staff comment topic
- Custom elements should follow the naming conventions of the standard taxonomy (camelCase, descriptive names)
Block Tagging vs. Detail Tagging
Block Tagging
Block tags wrap entire sections of text or tables, capturing the complete disclosure as a text block element. Required for:
- Each complete financial statement
- Each footnote in its entirety
- Each accounting policy within the significant accounting policies note
- Each table within the footnotes
- Cover page as a whole
Detail Tagging
Detail tags mark individual data points (numbers, dates, text) within the financial statements and notes. Required for:
- Each numeric value on the face of the financial statements
- Each numeric value in the footnote tables and schedules
- Per-share amounts, percentages, and dates in the footnotes
- Cover page data elements (entity name, CIK, fiscal year, form type, etc.)
- Dimensional qualifiers for disaggregated line items
XBRL Implementation Timeline
| Phase | Requirement | Status |
|---|---|---|
| Phase 1 (2009) | Large accelerated filers using US GAAP begin XBRL filing of financial statements. | Complete |
| Phase 2 (2010) | Accelerated filers begin XBRL filing. All filers begin detail tagging of financial statement notes and schedules. | Complete |
| Phase 3 (2011) | All remaining filers begin XBRL filing. | Complete |
| Inline XBRL (2019-2021) | Phased transition from XBRL attachments to Inline XBRL (iXBRL), where XBRL tags are embedded directly in the HTML filing. LAFs began June 2019; all filers by June 2021. | Complete |
| Cover Page Tagging (2019-2020) | Cover page data elements on Forms 10-K, 10-Q, 8-K, and others must be tagged in Inline XBRL. Phased in by filer category. | Complete |
Common XBRL Errors
| Error | Description | Impact |
|---|---|---|
| Incorrect Element Selection | Using a standard taxonomy element when a custom extension is needed, or vice versa. The element chosen must match the economic substance of the disclosure. | SEC comment letter, potential restatement of XBRL data |
| Calculation Inconsistencies | XBRL calculation linkbase totals do not match the values presented in the financial statements. Common with tax provisions, EPS, and segment data. | EDGAR validation warning or error |
| Missing Dimensions | Failure to apply required dimensional (axis/member) tags to line items that are broken out by segment, geography, or other dimension. | Data consumers cannot disaggregate the data correctly |
| Incorrect Period Type | Tagging a balance sheet item (instant) with a duration context, or tagging an income statement item (duration) with an instant context. | EDGAR validation error |
| Duplicate Custom Extensions | Creating a new custom element when an equivalent standard element already exists in the US GAAP taxonomy, or creating multiple custom elements for the same concept. | SEC staff comment, reduced data comparability |
| Incorrect Unit Assignment | Assigning the wrong unit (e.g., shares instead of monetary, or wrong currency code) to a numeric element. | EDGAR validation error |
| Block Tag Scope Errors | Block tags covering too much or too little text. Each block tag should encompass exactly the disclosure required by the corresponding element. | Data quality issue, may trigger SEC comment |
Validation Tools
- EDGAR Online Previewer -- Renders Inline XBRL filings as they will appear in the EDGAR viewer, allowing pre-submission review
- XBRL US EDGAR Dashboard -- Analyzes XBRL filing quality, identifies common errors, and provides comparability metrics
- Arelle -- Open-source XBRL processor that performs formula validation, calculation checks, and taxonomy consistency analysis
- SEC XBRL Rendering Engine -- The official rendering engine used by the SEC to display XBRL-tagged financial data on EDGAR
- Filing Agent Validation Suites -- Commercial filing agents (DFIN, Toppan Merrill, Workiva) provide proprietary validation tools integrated into their filing platforms