Fair Value Measurements--Fair Value Hierarchy (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Derivative assets, gross	$ 13,113		$ 46,010
Derivative liabilities, gross	(12,792)		(5,605)
Derivative assets (liabilities), net	321		40,405
Netting adjustments
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Derivative assets, gross	(4,558)	[1]	(2,977)	[1]
Derivative liabilities, gross	4,558	[1]	2,977	[1]
Recurring Fair Value Measurements
Fair Value Hierarchy for Financial Instruments Measured at Fair Value on a Recurring Basis [Line Items]
Derivative assets (liabilities), net	321		40,405
Derivative assets, net	8,555		43,033
Derivative liabilities, net	(8,234)		(2,628)
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	3,275		18,599
Derivative liabilities, gross	(6,576)		(4,425)
Derivative assets (liabilities), net	(3,301)		14,174
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	9,838		27,411
Derivative liabilities, gross	(6,216)		(1,180)
Derivative assets (liabilities), net	3,622		26,231
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	(4,558)	[1]	(2,977)	[1]
Derivative liabilities, gross	4,558	[1]	2,977	[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.