Retirement Benefits Plans - Fair Value of Other Postretirement Benefits Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value of Other Postretirement Benefits Plan Assets [Line Items]
Total pension plan assets	$ 4,447	$ 4,406
VEBA target trust allocation in diversified global equities	53.00%
VEBA target trust allocation in fixed income securities	47.00%
Other postretirement benefits plans [Member]
Fair Value of Other Postretirement Benefits Plan Assets [Line Items]
Total pension plan assets	$ 74	93	$ 116
Fixed income securities		18
United States treasuries		20
Cash equivalents	8	11
Common Collective Trusts, Fixed Income	66	44
Total other postretirement benefits plan assets	74	93
Other postretirement benefits plans [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Fair Value of Other Postretirement Benefits Plan Assets [Line Items]
Total pension plan assets	8	31
Fixed income securities		0
United States treasuries		20
Cash equivalents	8	11
Other postretirement benefits plans [Member] | Other observable inputs (Level 2) [Member]
Fair Value of Other Postretirement Benefits Plan Assets [Line Items]
Total pension plan assets	0	18
Fixed income securities		18
United States treasuries		0
Cash equivalents	0	0
Other postretirement benefits plans [Member] | Unobservable inputs (Level 3) [Member]
Fair Value of Other Postretirement Benefits Plan Assets [Line Items]
Total pension plan assets	0	0
Fixed income securities		0
United States treasuries		0
Cash equivalents	$ 0	$ 0