Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ (26,572)	$ (18,486)	$ 160,617	$ 44,398
Postretirement plans
Amortization of prior service cost, net of tax of ($28), ($42), ($85) and ($129), respectively	(85)	(71)	(255)	(212)
Amortization of net loss, net of tax of $8, $0, $23 and $3, respectively	23	2	71	4
Total postretirement plans	(62)	(69)	(184)	(208)
Comprehensive Income (Loss)	$ (26,634)	$ (18,555)	$ 160,433	$ 44,190