STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 49,238	$ 170,111
Postretirement benefits:
Prior service cost	92	0
Income tax benefit	(23)	0
Total comprehensive income	$ 49,307	$ 170,111