CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Comprehensive income:
Net income	$ 1,261	$ 1,148	$ 1,123
Pension and retiree medical benefits:
Net pension and retiree medical losses arising during the period, net of tax of $(2), $(2), and $(5), respectively	(6)	(3)	(8)
Amortization of losses included in net periodic benefit cost, net of tax of $3, $5, and $2, respectively	9	7	4
Total pension and retiree medical benefits, net of tax	3	4	(4)
Derivative instruments:
Reclassification of losses to net income, net of tax of $1, $2, and $2, respectively	3	3	4
Net fair value decrease, net of tax of $(2), $0, and $0, respectively	(5)	0	0
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax, Portion Attributable to Parent	(2)	3	4
Other comprehensive income	1	7	0
Comprehensive income	$ 1,262	$ 1,155	$ 1,123