CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Comprehensive income:
Net income	$ 291	$ 239
Pension and retiree medical benefits:
Amortization of losses included in net periodic benefit cost, net of tax of $0 and $1, respectively	1	1
Derivative instruments:
Reclassification of losses to net income, net of tax of $0 and $1, respectively	0	1
Other comprehensive income	1	2
Comprehensive income	$ 292	$ 241