CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net Income (Loss)	$ 306,828	$ (167,513)	$ (945,731)
Pension and postretirement plans:
Amortization of prior service cost, net of tax of ($114), ($176) and $0, respectively	(340)	(289)	0
Amortization of net (gain) loss, including settlement costs, net of tax of $2, $1,168 and $10,676, respectively	8	1,890	19,828
Current period change in fair value of pension and postretirement plans, net of tax of ($233), ($279), and $1,779, respectively	(693)	(459)	3,305
Total pension and postretirement plans	(1,025)	1,142	23,133
Comprehensive Income (Loss)	$ 305,803	$ (166,371)	$ (922,598)