CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Comprehensive income:
Net income	$ 588	$ 578	$ 552
Pension and retiree medical benefits:
Reclassification of loss (gain) to net income, net of tax of $1, $(1) and $—, respectively	2	(3)	0
Derivative instruments:
Reclassification of loss to net income, net of tax of $—, $— and $—, respectively	1	2	1
Total other comprehensive income (loss)	3	(1)	1
Total comprehensive income	$ 591	$ 577	$ 553