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Financial Instruments - Changes in Fair Value of Assets (Liabilities) Measured on Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Fair value at beginning of period $ (600) $ (10,000)
Realized and unrealized gains included in earnings 2,607 9,307
Settlements (907) 93
Fair value at end of period $ 1,100 $ (600)