Pension Benefits Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Defined Benefit Pension Plan, Postemployment Benefit Period	5 years
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	$ 62
Defined Contribution Plan, Cost	$ 36	$ 36	$ 41