Income Taxes - Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Income tax
Net UTB impacting the effective tax rate	$ 370		$ 370		$ 333
Interest and penalties related to unrecognized tax benefits, expense (benefit) recognized on a gross basis	5	$ 1	8	$ (3)
Interest and penalties related to unrecognized tax benefits, accrued on a gross basis	$ 56		$ 56		52
Increase (decrease) in effective income tax rate from prior reporting period to current reporting period (as a percent)	(3.60%)		(3.20%)
Impact of factors that decreased the effective tax rate from prior reporting period to current reporting period (as a percent)	(5.00%)
Impact of factors that increased the effective tax rate from prior reporting period to current reporting period (as a percent)	1.40%
Deferred Tax Assets, Valuation Allowance	$ 57		$ 57		$ 47
Reconciliation of Effective Income Tax Rate
Effective tax rate (as a percent)	26.00%	29.60%	25.00%	28.20%