Retirement Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Allocated Service Cost	$ 56	$ 125	$ 86
Defined Contribution Plan, Cost	7	5	4
Other Postretirement Benefits Cost (Reversal of Cost)	$ 3	$ 3	$ 4