Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 68,274	$ 29,481	$ 187,189	$ 62,884
Postretirement plans
Amortization of prior service cost, net of tax of ($28), ($42), ($56) and ($86), respectively	(85)	(71)	(170)	(141)
Amortization of net loss, net of tax of $8, $0, $15 and $2, respectively	23	2	47	3
Total postretirement plans	(62)	(69)	(123)	(138)
Comprehensive Income	$ 68,212	$ 29,412	$ 187,066	$ 62,746