STATEMENTS OF COMPREHENSIVE INCOME STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net income	$ 172,234	$ 162,995	$ 140,095
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Change in pension and other postretirement benefit plans liability, net of tax of $(848), $486, and $197, respectively	1,407	(778)	(314)
Other comprehensive income (loss), net of tax	1,407	(778)	(314)
Comprehensive income	$ 173,641	$ 162,217	$ 139,781