STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Comprehensive income:
Net income	$ 159,213	$ 152,157	$ 127,263
Other comprehensive income (loss)
Amortization of losses (gains) included in net periodic benefit cost, net of tax of $26, $(84), and $(260), respectively	44	(148)	(464)
Derivative instruments:
Reclassification of losses to net income, net of tax of $24, $80, and $97, respectively	39	139	172
Other comprehensive income	83	(9)	(292)
Comprehensive income	$ 159,296	$ 152,148	$ 126,971