STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Comprehensive income:
Net income	$ 152,157	$ 127,263	$ 129,852
Other comprehensive (loss) income
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, Net of Tax of $(84), $(260), and $0, respectively	(148)	(464)	0
Derivative instruments:
Reclassification of losses to net income, net of tax of $80, $97, and $96, respectively	139	172	172
Other comprehensive income	(9)	(292)	172
Comprehensive income	$ 152,148	$ 126,971	$ 130,024