Income Taxes - Tax Effected Operating Loss, Capital Loss, and Tax Credit Carryovers (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income tax
Net UTB impacting the effective tax rate	$ 526	$ 655	$ 526	$ 333
Interest and penalties related to unrecognized tax benefits, expense (benefit) recognized on a gross basis		12	20	$ 10
Interest and penalties related to unrecognized tax benefits, accrued on a gross basis	68	69	68
Deferred tax assets valuation allowance	81	$ 67	$ 81
Effective tax rate (as a percent)		23.40%	35.50%	28.30%
Increase (decrease) in effective income tax rate from prior reporting period to current reporting period (as a percent)		(12.10%)	7.20%
Statutory U.S. tax rate		21.00%	35.00%	35.00%
Provisional income tax expense (benefit) from enactment of Tax Cuts and Jobs Act of 2017	762
Tax Cuts and Jobs Act of 2017 measurement period adjustment		$ 176
Provisional transition tax expense (benefit) for accumulated foreign earnings from enactment of Tax Cuts and Jobs Act of 2017	745	97
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	$ 17	$ 79
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 long term income taxes payable		$ 649