CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Comprehensive income:
Net income	$ 984,485	$ 1,021,306	$ 948,234
Pension and retiree medical benefits:
Net pension and retiree medical benefit (losses) gains arising during the period, net of tax of $(5,026), $(4,687), and $1,746, respectively	(7,906)	(7,517)	1,408
Amortization of losses included in net periodic benefit cost, net of tax of $2,249, $2,159, and $4,151, respectively	3,526	3,495	3,306
Total pension and retiree medical benefits, net of tax	(4,380)	(4,022)	4,714
Derivative instruments:
Net fair value (decrease) increase, net of tax of $(46), $(103), and $17, respectively	(70)	(163)	12
Reclassification of losses to net income, net of tax of $1,810, $1,493, and $2,541, respectively	2,836	2,288	1,476
Total derivative instruments, net of tax	2,766	2,125	1,488
Marketable securities:
Net fair value increase, net of tax of $0, $21, and $117, respectively	0	33	176
Other comprehensive (loss) income	(1,614)	(1,864)	6,378
Comprehensive income	$ 982,871	$ 1,019,442	$ 954,612