Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 118,915	$ 33,403
Postretirement plans
Amortization of prior service cost, net of tax of ($28) and ($43), respectively	(85)	(71)
Amortization of net loss, net of tax of $8 and $1, respectively	23	2
Total postretirement plans	(62)	(69)
Comprehensive Income	$ 118,853	$ 33,334