CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 13,886	$ (5,822)	$ 16,178	$ 15,185	$ 19,005	$ 18,824	$ 7,006	$ 27,786	$ 39,427	$ 72,621	$ (45,835)
Other comprehensive income (loss):
Other post-retirement benefits income (loss), net of tax									529	(52)	2,196
Total other comprehensive income (loss), net of tax									529	(52)	2,196
Comprehensive income (loss)									$ 39,956	$ 72,569	$ (43,639)