Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017
Income Tax Contingency [Line Items]
Tax expense	$ 159		$ 30	$ 211	$ 136
Income before income taxes	513		162	809	591
Impact on income tax expense, discrete tax adjustments	(31)			(10)
Foreign tax credits on dividends	25	$ (25)
Provisional income tax expense (benefit)	11			14
Other adjustments, amount	5		$ 12	4	$ 23
Provisional liability	91			91			$ 77
Provision for undistributed foreign earnings amount	$ 10			$ 10			19
Tax credit carryforwards, foreign							$ 750
Scenario, Forecast
Income Tax Contingency [Line Items]
Change in valuation allowance (up to)						$ 25