CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Comprehensive income:
Net income	$ 238	$ 265	$ 553	$ 556
Pension and retiree medical benefits:
Net pension and retiree medical gains arising during the period, net of tax of $0, $0, $1 and $0, respectively	1	0	3	0
Amortization of losses included in net periodic benefit cost, net of tax of $0, $1, $0 and $1, respectively	0	1	1	2
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Reclassification Adjustment, before Tax	1	1	4	2
Derivative instruments:
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax	(10)	0	(17)	0
Reclassification of losses to net income, net of tax of $0	1	1	2	1
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax, Portion Attributable to Parent	(9)	1	(15)	1
Other comprehensive (loss) income	(8)	2	(11)	3
Comprehensive income	$ 230	$ 267	$ 542	$ 559