Fair Value Measurements - Derivative Instrument Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets
Derivative instruments, at fair value	$ 270,288	$ 241,952
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	270,288	241,952
Liabilities
Derivative instruments, at fair value	146,061	139,089
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	270,288	241,952
Liabilities
Derivative instruments, at fair value	$ 146,061	$ 139,089