Fair Value Measurements--Fair Value Hierarchy (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets, gross	$ 251,863		$ 13,113
Derivative liabilities, Gross assets	(309)		(12,792)
Derivative assets (liabilities), gross	251,554		321
Derivative assets, net	251,631		8,555
Derivative liabilities, net	(77)		(8,234)
Recurring Fair Value Measurements [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets, gross	232	[1]	4,558	[1]
Derivative liabilities, Gross assets	(232)	[1]	(4,558)	[1]
Derivative assets (liabilities), gross	0	[1]	0	[1]
Derivative assets, net	251,631		8,555
Derivative liabilities, net	(77)		(8,234)
Derivative assets (liabilities), net	251,554		321
Recurring Fair Value Measurements [Member] | Significant other observable inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets, gross	56,696		3,275
Derivative liabilities, Gross assets	(309)		(6,576)
Derivative assets (liabilities), gross	56,387		(3,301)
Recurring Fair Value Measurements [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets, gross	195,167		9,838
Derivative liabilities, Gross assets	0		(6,216)
Derivative assets (liabilities), gross	$ 195,167		$ 3,622

[1] Amounts represent the impact of master netting agreements that allow us to net settle positive and negative positions with the same counterparty. Positive and negative positions with a counterparty are netted on the balance sheet only to the extent that they relate to the same current versus noncurrent classification.