its in xbrl 2.1 specification
http://www.xbrl.org/Specification/XBRL-RECOMMENDATION-2003-12-31+Corrected-Errata-2005-04-25.htm#_4.6.1
<context id="BalanceAsOf_20120531">
<entity>
<identifier scheme="http://www.sec.gov/CIK">0000866729</identifier>
</entity>
<period>
<instant>2012-05-31</instant>
</period>
</context>
<context id="BalanceAsOf_20120531_AccumulatedOtherComprehensiveIncomeMember">
<entity>
<identifier scheme="http://www.sec.gov/CIK">0000866729</identifier>
<segment>
<xbrldi:explicitMember dimension="us-gaap:StatementEquityComponentsAxis">us-gaap:AccumulatedOtherComprehensiveIncomeMember</xbrldi:explicitMember>
</segment>
</entity>
<period>
<instant>2012-05-31</instant>
</period>
</context>
<unit id="USD">
<measure>iso4217:USD</measure>
</unit>
<us-gaap:AssetsCurrent contextRef="BalanceAsOf_20120531" unitRef="USD" decimals="-5">929900000.0</us-gaap:AssetsCurrent>
<us-gaap:Assets contextRef="BalanceAsOf_20120531" unitRef="USD" decimals="-5">1670300000.0</us-gaap:Assets>
<us-gaap:StockholdersEquity contextRef="BalanceAsOf_20120531_AccumulatedOtherComprehensiveIncomeMember" unitRef="USD" decimals="-5">-74200000.0</us-gaap:StockholdersEquity>
for example; consider these two context; ids
needs to be unique with respect to their content(With respect to period
entity
; and entity
with its sub elements; identifier
and segment
if present)
we will use its id
to map financial facts. Here in example AssetsCurrent
refers to unit--> whose id
is USD
and context information--> whose id is BalanceAsOf_20120531
; and detail of this this context id can be found in context
element.
here everything should be unique other wise it will create duplicate facts/contexts.
You can see here that; both context has same period but so why can't their ids
? because one context has addition detail segment
information.
(i am not going in detail here; if you want to know; how it is detecting duplicate/error consider example no 25 and 26 from the link.)
in other way you can also view facts/values with three axis as CIK
period
and xmlname
;and this uniqueness makes them comparing with other company's facts with these axis.