STATEMENTS OF COMPREHENSIVE INCOME STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net income	$ 140,095	$ 119,030	$ 109,790
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Change in pension and other postretirement benefit plans liability, net of tax of $197, $(483), and $1,244, respectively	(314)	773	(1,781)
Other comprehensive loss	(314)	773	(1,781)
Comprehensive income	$ 139,781	$ 119,803	$ 108,009