Fair Value Disclosures - Summary of Changes in assets (liabilities) - Level 3 (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance of Level 3 at beginning of period	$ (5,449)	$ (9,973)
Derivative deferred premiums - purchases	0	(516)
Derivative deferred premiums - settlements	2,571	5,040
Balance of Level 3 at end of period	$ (2,878)	$ (5,449)