CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Comprehensive income:
Net income	$ 463,491	$ 466,802	$ 455,188
Pension and retiree medical benefits:
Net pension and retiree medical losses arising during the period, net of tax of $(138), $0, and $0	(223)	0	0
Amortization of losses included in net periodic benefit cost, net of tax of $2, $0, and $0, respectively	3	0	0
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, before Tax	(220)	0	0
Derivative instruments:
Net fair value decrease, net of tax of $0, $(20), and $(43), respectively	0	(30)	(72)
Reclassification of losses (gains) to net income, net of tax of $648, $39, and $(287), respectively	1,056	72	(468)
Total derivative instruments, net of tax	1,056	42	(540)
Other comprehensive income (loss)	836	42	(540)
Comprehensive income	$ 464,327	$ 466,844	$ 454,648