Employee Retirement Benefits - Net Periodic Income Cost Recognizedf for OCI (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Total	$ (44)	$ 21	$ 42
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior service benefit (net of $3 tax expense)	0	4	0
Net gains (losses) (net of $23 tax expense and $16 and $27 tax benefit, respectively)	37	(26)	(44)
Total	37	(22)	(44)
Tax effects on components of net periodic income (cost) recognized in OCI [Abstract]
Prior service benefit (cost)	0	3	0
Net gains (losses)	$ 23	$ (16)	$ (27)