Fair Value Measurements - Fair Value Hierachy (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Derivative assets, gross	$ 47,152		$ 40,252
Derivative liabilities, gross	(6,433)		(12,259)
Derivative assets (liabilities), net	40,719		27,993
Netting adjustments
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Derivative assets, gross	(4,634)	[1]	(10,627)	[1]
Derivative liabilities, gross	4,634	[1]	10,627	[1]
Recurring Fair Value Measurements
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Derivative assets, net	42,518		29,625
Derivative liabilities, net	(1,799)		(1,632)
Total derivative assets (liabilities), net	40,719		27,993
Recurring Fair Value Measurements | Significant other observable inputs (Level 2)
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Derivative assets, gross	21,623		33,956
Derivative liabilities, gross	(6,433)		(11,012)
Derivative assets (liabilities), net	15,190		22,944
Recurring Fair Value Measurements | Significant unobservable inputs (Level 3)
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Derivative assets, gross	25,529		6,296
Derivative liabilities, gross	0		(1,247)
Derivative assets (liabilities), net	25,529		5,049
Recurring Fair Value Measurements | Netting adjustments
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Derivative assets, gross	(4,634)	[1]	(10,627)	[1]
Derivative liabilities, gross	4,634	[1]	10,627	[1]
Derivative assets (liabilities), net	$ 0	[1]	$ 0	[1]

[1] Amounts represent the impact of master netting agreements that allow us to net settle positive and negative positions with the same counterparty.