Derivative Financial Instruments - Commodity Derivatives (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Realized and Unrealized Gains (Losses) Related to Derivatives [Roll Forward]
Total gain (loss) on derivatives			$ 8,078	$ (45,893)
Derivative cash settlements paid (received)			(77,526)	9,620
Not designated as hedging instrument | Commodity contract
Realized and Unrealized Gains (Losses) Related to Derivatives [Roll Forward]
Beginning fair value of interest rate swaps	$ 133,242	$ 5,397	153,099	17,673
Ending fair value of interest rate swaps	82,556	(20,026)	82,556	(20,026)
Not designated as hedging instrument | Commodity contract | Oil
Realized and Unrealized Gains (Losses) Related to Derivatives [Roll Forward]
Total gain (loss) on derivatives	(12,649)	(33,770)	945	(46,030)
Derivative cash settlements paid (received)	(27,364)	6,244	(59,564)	8,800
Not designated as hedging instrument | Commodity contract | Natural gas
Realized and Unrealized Gains (Losses) Related to Derivatives [Roll Forward]
Total gain (loss) on derivatives	(848)	2,337	6,038	(1,289)
Derivative cash settlements paid (received)	$ (9,825)	$ (234)	$ (17,962)	$ 820