Income Taxes - Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Income tax
Net UTB impacting the effective tax rate	$ 348		$ 333
Interest and penalties related to unrecognized tax benefits, expense (benefit) recognized on a gross basis	3	$ (4)
Interest and penalties related to unrecognized tax benefits, accrued on a gross basis	$ 55		52
Increase (decrease) in effective income tax rate from prior reporting period to current reporting period (as a percent)	(3.10%)
Impact of factors that decreased the effective tax rate from prior reporting period to current reporting period (as a percent)	(3.60%)
Impact of factors that increased the effective tax rate from prior reporting period to current reporting period (as a percent)	0.50%
Deferred Tax Assets, Valuation Allowance	$ 51		$ 47
Reconciliation of Effective Income Tax Rate
Effective tax rate (as a percent)	23.70%	26.80%