Accounting for Derivative Instruments - Gains (Losses) on Derivative Instruments (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Current period settlements on derivatives	$ (3,479)	$ 1,040	$ 26,801
Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on derivatives, net	(14,535)	6,567	(21,485)
Current period settlements on derivatives	(3,479)	1,040	26,801
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on derivatives, net	(8,713)	1,345	(1,074)
Current period settlements on derivatives	(6,289)	1,833	7,782
Not Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on derivatives, net	0	84	(17,942)
Current period settlements on derivatives	0	(1,180)	18,707
Ineffectiveness gain (loss)		(288)	930
Designated as Hedging Instrument | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on derivatives, net	(5,822)	5,138	(2,469)
Current period settlements on derivatives	2,810	387	312
Designated as Hedging Instrument | Cash Flow Hedging | Affiliated Entity
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on derivatives, net	203	1,509	506
Current period settlements on derivatives	$ 315	$ (1,780)	$ 87