Derivative Instruments Summary of Outstanding Contracts with Respect to Diesel Fuel (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative [Line Items]
Non-cash gain (loss) on derivatives, net			$ (12,013)	$ 65,481
Gain on natural gas derivatives, net	$ (2,025)	$ 8,602	(4,536)	83,482
Diesel Fuel [Member]
Derivative [Line Items]
Cash received (paid) on derivatives, net	0	603	0	1,522
Non-cash gain (loss) on derivatives, net	0	740	0	(2,989)
Gain on natural gas derivatives, net	$ 0	$ 1,343	$ 0	$ (1,467)