Fair Value Measurements - Derivative Instrument Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Assets
Derivative instruments, at fair value	$ 134,698	$ 417,397
Fair value on a recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Derivative instruments, at fair value	0	0
Liabilities
Derivative instruments, at fair value	0	0
Fair value on a recurring basis | Significant other observable inputs (Level 2)
Assets
Derivative instruments, at fair value	134,698	417,397
Liabilities
Derivative instruments, at fair value	67,871	23,434
Fair value on a recurring basis | Significant unobservable inputs (Level 3)
Assets
Derivative instruments, at fair value	0	0
Liabilities
Derivative instruments, at fair value	0	0
Fair value on a recurring basis | Fair value
Assets
Derivative instruments, at fair value	134,698	417,397
Liabilities
Derivative instruments, at fair value	$ 67,871	$ 23,434