LEI Taxonomy Example

This is an analysis of the Legal Entity Identifier (LEI) taxonomy, staged in this vault as raw source material under docs/Sources/xbrl/lei-taxonomy-REC-2020-07-02/ (open it in an XBRL processor such as Arelle to follow along). It's a simple, real-world example of how XBRL taxonomies work.

What Is an LEI?

A Legal Entity Identifier is a 20-character alphanumeric code (ISO 17442) that uniquely identifies legal entities participating in financial transactions. Format: [0-9A-Z]{18}[0-9]{2} (18 alphanumeric + 2 check digits).

Package Structure

This is a taxonomy package:

lei-taxonomy-REC-2020-07-02/
├── META-INF/
│   ├── taxonomyPackage.xml    ← Package manifest
│   └── catalog.xml            ← URL remapping
├── lei.xsd                    ← Main schema (entry point 1)
├── lei-required.xsd           ← Required LEI schema (entry point 2)
├── lei-formula-functions.xsd  ← Formula functions (entry point 3)
├── lei-label.xml              ← Label linkbase
├── lei-formula.xml            ← Formula linkbase (validation rules)
├── lei-required-formula.xml   ← Additional formula rules
└── lei-formula-functions.xml  ← Custom XPath functions

The Schema (lei.xsd)

Defines one concept: LEI

<xsd:element name="LEI"
  id="lei_LEI"
  substitutionGroup="xbrli:item"
  nillable="true"
  xbrli:periodType="duration"
  type="lei:leiItemType" />

And a custom data type with validation:

<xsd:complexType name="leiItemType">
  <xsd:restriction base="xbrli:stringItemType">
    <xsd:pattern value="[0-9A-Z]{18}[0-9]{2}"/>
    <xsd:length value="20"/>
  </xsd:restriction>
</xsd:complexType>

This enforces that any reported LEI must be exactly 20 characters matching the pattern.

Also defines a domain element (LEIDomain) and a simple type (leiType) for use in dimensional contexts.

The Label Linkbase (lei-label.xml)

Provides two labels for the LEI concept:

• Standard label: "Legal Entity Identifier"
• Documentation label: "Legal Entity Identifier (LEI) for the reporting entity."

This is a minimal example of a label linkbase.

The Formula Linkbase (lei-formula.xml)

Contains five formula assertions -- a rich example of validation:

Assertion	What It Validates
lei-fact-checksum	LEI fact values have valid checksums
lei-fact-checksum-digits	Check digits are not 00, 01, or 99
lei-identifier-checksum	Entity identifier LEIs have valid checksums
lei-identifier-checksum-digits	Entity identifier check digits are valid
lei-identifier-format	Entity identifier LEIs have correct format

Each assertion uses:

• Variable filters to select relevant facts (by data type or entity scheme)
• Custom functions (lei-fn:validate-checksum, lei-fn:validate-format)
• Error messages with dynamic content: "The value '{ $v }' is not a valid Legal Entity Identifier (invalid checksum)"

The Three Entry Points

From the taxonomyPackage.xml:

1. Legal Entity Identifier (lei.xsd) -- the standard concept and validation
2. Legal Entity Identifier (required primary identifier) (lei-required.xsd) -- enforces that ALL facts use an LEI as the entity identifier
3. LEI formula functions (lei-formula-functions.xsd) -- just the validation functions for reuse

What This Example Teaches

1. Simple taxonomy structure: one concept, one schema, a few linkbases
2. Custom data types: how to restrict values with patterns and lengths
3. Formula validation: real-world checksum validation beyond simple arithmetic
4. Multiple entry points: different levels of enforcement from the same taxonomy
5. Package structure: how META-INF, schemas, and linkbases are organised