Accumulated Other Comprehensive Income Reclassification out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017		Sep. 30, 2016
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Prior service credits, Total before tax	[1],[2]	$ 1	$ 1	$ 2		$ 2
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Prior service credits, Total before tax	[1]	1	1	2		2
Tax benefit / (expense)	[1]	(1)	0	(1)		(1)
Net of Tax	[1]	$ 0	$ 1	$ 1	[3]	$ 1	[3]

[1] Amounts in parenthesis 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 8 for additional details).

[3] Amounts are net of tax.