CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 333,146	$ (521,388)	$ (713,685)
Postretirement benefits:
Prior service cost	(1,769)	0	0
Income tax benefit	437	0	0
Total comprehensive income (loss)	$ 331,814	$ (521,388)	$ (713,685)