CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Comprehensive income:
Net income	$ 133,722	$ 111,546
Amortization of losses included in net periodic benefit cost, net of tax of $1, and $1, respectively	2	1
Derivative instruments:
Reclassification of losses to net income, net of tax of $98 and $152, respectively	300	246
Other comprehensive income	302	247
Comprehensive income	$ 134,024	$ 111,793