Income Taxes - Tax Effected Operating Loss, Capital Loss, and Tax Credit Carryovers (Details) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2017
Income tax
Net UTB impacting the effective tax rate	$ 518	$ 526		$ 526
Interest and penalties related to unrecognized tax benefits, expense (benefit) recognized on a gross basis	9		$ 3
Interest and penalties related to unrecognized tax benefits, accrued on a gross basis	69	68		68
Deferred Tax Assets, Valuation Allowance	$ 85	81		$ 81
Effective tax rate (as a percent)	37.20%		23.70%
Increase (decrease) in effective income tax rate from prior reporting period to current reporting period (as a percent)	13.50%
Impact of factors that decreased the effective tax rate from prior reporting period to current reporting period (as a percent)	14.30%
Impact of factors that increased the effective tax rate from prior reporting period to current reporting period (as a percent)	27.80%
Statutory U.S. tax rate	21.00%			35.00%
Provisional income tax expense (benefit) from enactment of Tax Cuts and Jobs Act of 2017	$ 217	762
Provisional transition tax expense (benefit) for accumulated foreign earnings from enactment of Tax Cuts and Jobs Act of 2017	132	745
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	$ 85	$ 17
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	$ 144
Tax Cuts and Jobs Act of 2017 transition tax long term income taxes payable	$ 733