Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2017
Income Tax Contingency [Line Items]
Tax expense	$ 19		$ 36	$ 52	$ 106
Income before income taxes	183		190	296	429
Favorable impact on income tax expense, discrete tax adjustments	28		$ 9	21	$ 11
Foreign tax credits on dividends	25
Transition tax expense (benefit)	(4)	$ 7
Provisional liability	80			80			$ 77
Provisional liability, increase (decrease) during period	(4)	$ 7
Provision for undistributed foreign earnings amount	$ 8			$ 8			19
Tax credit carryforwards, foreign							$ 750
Scenario, Forecast
Income Tax Contingency [Line Items]
Change in valuation allowance (up to)						$ 20