CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Comprehensive income:
Net income	$ 186,077	$ 173,607	$ 398,210	$ 376,825
Amortization of losses (gains) included in net periodic benefit cost, net of tax of $1, $0, $3 and $(134), respectively	1	0	3	(217)
Derivative instruments:
Net fair value increase, net of tax of $0, $(1), $0, and $1, respectively	0	(1)	0	1
Reclassification of losses to net income, net of tax of $150, $162, $455, and $486, respectively	257	266	753	792
Other comprehensive income	258	265	756	576
Comprehensive income	$ 186,335	$ 173,872	$ 398,966	$ 377,401