Supplemental Financial Information - Amounts Reclassified out of AOCI (Details) - Reclassification out of Accumulated Other Comprehensive Income - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net actuarial losses of defined benefit plans
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassified from other comprehensive income (loss), before tax	[1]	$ 11	$ 16	$ 23	$ 34
Tax Effect		(2)	(5)	(5)	(11)
Recognized within Net income, net of taxes		9	11	18	23
Prior service credit of defined benefit plans
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassified from other comprehensive income (loss), before tax	[1]	(1)	(2)	(2)	(3)
Tax Effect		0	1	0	1
Recognized within Net income, net of taxes		$ (1)	$ (1)	$ (2)	$ (2)

[1] Detailed in Note 6.