Income Taxes - Tax Cuts and Jobs Act ("Act") (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Income Taxes
Statutory U.S. Federal tax rate (as a percent)		21.00%	35.00%
Provisional amount expected to be offset by available foreign tax credits	$ 314		$ 331
Provisional amount expected to be offset by available foreign tax credits primarily attributable to state taxes	2		$ 2
Change to income tax expense due to impact of foreign tax credits	0
Adjustment of provisional amount of transition tax	17
All adjustments of provisional amount of transition tax	$ 0	$ 0