CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Comprehensive income:
Net income	$ 173,607	$ 173,081	$ 376,825	$ 382,547
Amortization of losses (gains) included in net periodic benefit cost, net of tax of $(1), $0, $133 and $0, respectively	0	0	(217)	0
Derivative instruments:
Net fair value increase, net of tax of $(1), $(10), $1, and $(10), respectively	(1)	(17)	1	(14)
Reclassification of losses (gains) to net income, net of tax of $162, $5, $486 and $(123), respectively	266	19	792	(192)
Other comprehensive income (loss)	265	2	576	(206)
Comprehensive income	$ 173,872	$ 173,083	$ 377,401	$ 382,341