Income Taxes - Quarter End (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Income tax
Net UTB impacting the effective tax rate	$ 356		$ 356		$ 369
Interest and penalties related to unrecognized tax benefits, expense (benefit) recognized on a gross basis	11	$ 5	8	$ 3
Interest and penalties related to unrecognized tax benefits, accrued on a gross basis	$ 50		$ 50		45
Increase (decrease) in effective income tax rate from prior reporting period to current reporting period (as a percent)	(1.10%)		(0.80%)
Impact of factors that decreased the effective tax rate from prior reporting period to current reporting period (as a percent)	(6.30%)		(4.00%)
Impact of factors that increased the effective tax rate from prior reporting period to current reporting period (as a percent)	5.20%		3.20%
Deferred Tax Assets, Valuation Allowance	$ 39		$ 39		$ 31
Reconciliation of Effective Income Tax Rate
Effective tax rate (as a percent)	28.50%	29.60%	28.30%	29.10%