STATEMENTS OF COMPREHENSIVE INCOME STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net income	$ 186,749	$ 172,234	$ 162,995
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Change in pension and other postretirement benefit plans liability, net of tax of $479, $(848), and $486, respectively	(1,435)	1,407	(778)
Other comprehensive income (loss), net of tax	(1,435)	1,407	(778)
Comprehensive income	$ 185,314	$ 173,641	$ 162,217