Accumulated Other Comprehensive Income Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from AOCI, Before Tax	[1]	$ 2	$ 1	$ (23)	$ 2
Reclassification from AOCI, Tax benefit / (expense)	[1]	(1)	(1)	(1)	(1)
Reclassification from AOCI, Net of tax	[1]	1	0	(24)	1
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from AOCI, Before Tax	[1],[2]	$ 2	$ 1	2	2
Accounting Standards Update 2016-01 [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from AOCI, Before Tax	[1],[2]			1	0
Accounting Standards Update 2018-02 [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from AOCI, Before Tax	[1],[2]			$ (26)	$ 0

[1] Amounts in parentheses indicate debits to the income statement.

[2] This accumulated other comprehensive income component is included in the computation of net periodic pension and retiree health and welfare cost (see Note 9 for additional details).