Income Taxes - Tax Effected Operating Loss, Capital Loss, and Tax Credit Carryovers (Details) - USD ($)	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income tax
Net UTB impacting the effective tax rate	$ 493,000,000		$ 526,000,000		$ 493,000,000		$ 526,000,000
Interest and penalties related to unrecognized tax benefits, expense (benefit) recognized on a gross basis	(6,000,000)			$ 5,000,000	3,000,000	$ 8,000,000
Interest and penalties related to unrecognized tax benefits, accrued on a gross basis	60,000,000		68,000,000		60,000,000		68,000,000
Deferred Tax Assets, Valuation Allowance	$ 83,000,000		81,000,000		$ 83,000,000		$ 81,000,000
Effective tax rate (as a percent)	20.80%			26.00%	25.60%	25.00%
Increase (decrease) in effective income tax rate from prior reporting period to current reporting period (as a percent)	(5.20%)				0.60%
Statutory U.S. tax rate					21.00%		35.00%
Provisional income tax expense (benefit) from enactment of Tax Cuts and Jobs Act of 2017	$ 0	$ 217,000,000	762,000,000
Provisional transition tax expense (benefit) for accumulated foreign earnings from enactment of Tax Cuts and Jobs Act of 2017	0	132,000,000	745,000,000
Provisional expense (benefit) from remeasurement of deferred tax assets and liabilities due to change in tax rate from enactment of Tax Cuts and Jobs Act of 2017	0	$ 85,000,000	$ 17,000,000
Period of time over which TCJA transition tax on certain unrepatriated earnings of foreign subsidiaries may be paid					8 years
Tax Cuts and Jobs Act of 2017 transition tax in current accrued income taxes payable	74,000,000				$ 74,000,000
Tax Cuts and Jobs Act of 2017 transition tax long term income taxes payable	$ 733,000,000				$ 733,000,000