The XBRL-CONF-CR5-2012-01-24 conformance suite contains the following testcase variation, numbered 304-26, which is invalid XBRL:
<?xml version="1.0"?>
<!-- Copyright 2003 XBRL International Inc. See www.xbrl.org/legal. All Rights Reserved. -->
<xbrl xmlns="http://www.xbrl.org/2003/instance" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:my="http://www.someCompany.com" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.someCompany.com 304-03-monetaryItemTypeUnitsRestrictions.xsd">
<link:schemaRef xlink:href="304-03-monetaryItemTypeUnitsRestrictions.xsd" xlink:type="simple" />
<my:assets unitRef="u1" contextRef="c1" precision="1">120</my:assets>
<context id="c1">
<entity>
<identifier scheme="http://www.someCompany.com">example</identifier>
</entity>
<period>
<instant>2003-02-27</instant>
</period>
</context>
<unit xmlns:ISO4217="http://www.xbrl.org/2003/iso4217" id="u1">
<divide>
<unitNumerator>
<measure>ISO4217:USD</measure>
</unitNumerator>
<unitDenominator>
<measure>xbrli:shares</measure>
</unitDenominator>
</divide>
</unit>
</xbrl>
Can someone tell me why this is invalid? I am guessing that it has to do with the unit's monetary item type and the element, though the XBRL spec does not really call out anything specific regarding monetary item types and unit numerators and denominators. This XBRL reads as though the instance is specifying $120 USD/share posted on 27 Feb 2003. Can someone point me to the part of the XBRL spec that discusses why this is invalid?
I found it. The issue is that monetary item types cannot use complex units.
I looked at the EarningsPerShareBasic type in the 2013 US GAAP taxonomy, and it uses a decimalItemType, rather than a monetaryItemType. I suppose that this makes sense, because the monetary type needed to say something like "$120/share" would be specified in the complex unit (as in USD/share).
From all of this, I conclude that what the 304-26 conformance test is testing is that monetary item types (which is what the "assets" fact in the test uses) cannot use complex types for its units. According to Table 3 in section 4.8.2 of the XBRL spec, this also holds true for shares item types.