STATEMENTS OF COMPREHENSIVE INCOME STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net income	$ 119,030	$ 109,790	$ 99,195
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Change in pension and other postretirement benefit plans liability, net of tax of $(483), $1,244, and $0, respectively	773	(1,781)	0
Other comprehensive loss	773	(1,781)	0
Comprehensive income	$ 119,803	$ 108,009	$ 99,195