Derivative Instruments Derivative Instruments - Reconciles Net Amounts Derivative Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative assets	$ 2,603	$ 4,061
Noncurrent derivative assets	0	0
Derivative assets, Net amounts of assets on balance sheet	2,603	4,061
Derivative liabilities	0	(59,489)
Noncurrent derivative liabilities	0	0
Derivative liability, Net amounts of liabilities on balance sheet	0	(59,489)
Total derivative assets (liabilities), net	$ 2,603	$ (55,428)