Income Taxes - Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Income tax
Net UTB impacting the effective tax rate	$ 384		$ 384		$ 333
Interest and penalties related to unrecognized tax benefits, expense (benefit) recognized on a gross basis	8	$ 11	16	$ 8
Interest and penalties related to unrecognized tax benefits, accrued on a gross basis	$ 64		$ 64		52
Increase (decrease) in effective income tax rate from prior reporting period to current reporting period (as a percent)	(0.20%)		(2.20%)
Impact of factors that decreased the effective tax rate from prior reporting period to current reporting period (as a percent)	(4.00%)		(3.00%)
Impact of factors that increased the effective tax rate from prior reporting period to current reporting period (as a percent)	3.80%		0.80%
Deferred Tax Assets, Valuation Allowance	$ 62		$ 62		$ 47
Reconciliation of Effective Income Tax Rate
Effective tax rate (as a percent)	28.30%	28.50%	26.10%	28.30%