CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Other Comprehensive Income [Abstract]
Net income	$ 11,894	$ 10,159	$ 142,623	$ 57,881
Employee benefit plans:
Amortization of and adjustments to employee benefit costs	39	(51)	(11)	(1,069)
Income tax	0	0	0	0
Total other comprehensive income (loss), net of tax	39	(51)	(11)	(1,069)
Total comprehensive income	$ 11,933	$ 10,108	$ 142,612	$ 56,812