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Financial Instruments - Restricted Cash Balances (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Cash collateral for amounts under:    
Restricted cash $ 11,408 $ 97,878
Credit Agreement [Member]
   
Cash collateral for amounts under:    
Restricted cash   88,000
Letter of credit [Member]
   
Cash collateral for amounts under:    
Restricted cash 8,906 7,376
Funds restricted for legal matter [Member]
   
Cash collateral for amounts under:    
Restricted cash $ 2,502 $ 2,502