Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018
Income taxes
Income tax expense (benefit)	$ 19	$ 74	$ (65)	$ 134
Net discrete tax benefit	$ (16)		$ (114)	$ (8)
Estimated annual effective tax rate excluding impact of discrete items			20.80%	28.60%
Tax Cuts and Jobs Act (Tax Act)
Income taxes
Income tax expense (benefit)					$ 176
Tax Cuts and Jobs Act (Tax Act) | Taxation of Unremitted Foreign Earnings
Income taxes
Income tax expense (benefit)					$ 193
Period of tax payments on unremitted foreign earnings (in years)					8 years
Tax Cuts and Jobs Act (Tax Act) | Dividends Received Deduction for Certain Foreign Tax Credits
Income taxes
Income tax expense (benefit)					$ (113)
Tax Cuts and Jobs Act (Tax Act) | Remeasurement of Deferred Tax Assets and Liabilities
Income taxes
Income tax expense (benefit)					$ 96