Fair Value Measurements - Derivative Instrument Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets
Trading securities	$ 0	$ 286,396
Derivative instruments, at fair value	67,555	33,053
Fair value on a recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Trading securities		0
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
Trading securities		286,396
Derivative instruments, at fair value	67,555	33,053
Liabilities
Derivative instruments, at fair value	71,374	257,943
Fair value on a recurring basis | Significant unobservable inputs (Level 3)
Assets
Trading securities		0
Derivative instruments, at fair value	0	0
Liabilities
Derivative instruments, at fair value	0	0
Fair value on a recurring basis | Fair value
Assets
Trading securities		286,396
Derivative instruments, at fair value	67,555	33,053
Liabilities
Derivative instruments, at fair value	$ 71,374	$ 257,943