CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 48,539	$ (127,700)	$ 17,941	$ 111,961
Postretirement benefits:
Prior service cost	91	0	276	0
Income tax benefit	(22)	0	(68)	0
Total comprehensive income (loss)	$ 48,608	$ (127,700)	$ 18,149	$ 111,961