CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 35,439	$ (83,891)	$ 28,169	$ 61,092	$ 13,886	$ (5,822)	$ 16,178	$ 15,185	$ 40,809	$ 39,427	$ 72,621
Other comprehensive income (loss):
Other post-retirement benefits income (loss), net of tax									(2,091)	529	(52)
Total other comprehensive income (loss), net of tax									(2,091)	529	(52)
Comprehensive income									$ 38,718	$ 39,956	$ 72,569