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Debt (Details 4) (USD $) In Millions, unless otherwise specified | Mar. 31, 2012 | Dec. 31, 2011
--- | --- | ---
Debt |  |
Outstanding letters of credit | $ 42 | $ 33

X
- Details
Name: | us-gaap_DebtDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + Details
Name: | us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant