Derivative Financial Instruments - Summary of Pre-tax Realized and Unrealized Gain (Loss) on Derivative Instruments (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017
Derivative [Line Items]
Unrealized (loss) gain on commodity derivatives		$ 4,133
Total gain on commodity derivatives	$ 4,650	12,149
Commodity Derivative Contract
Derivative [Line Items]
Unrealized (loss) gain on commodity derivatives	(4,234)	4,133
Total gain on commodity derivatives	4,650	12,149
Commodity Derivative Contract | Natural Gas
Derivative [Line Items]
Realized gain on commodity derivatives-natural gas	$ 8,884	$ 8,016