Fair Value Measurements - Rollforward of significant unobservable inputs (Details) (USD $)
In Thousands, unless otherwise specified
6 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Balance, at beginning of period $ 0 $ 81,995
Total gains or losses    
Realized gain included in earnings 0 15,587
Reclassification of previous unrealized gain out of other comprehensive income 0 (14,654)
Unrealized loss included in other comprehensive income, net 0 (1,713)
Purchases, issuances, settlements, sales and accretion    
Purchases 0 0
Issuances 0 0
Settlements 0 (82,061)
Accretion of discount included in earnings 0 846
Balance, at end of period $ 0 $ 0