Income Taxes - Quarter End (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Income tax
Net UTB impacting the effective tax rate	$ 383		$ 383		$ 369
Interest and penalties related to unrecognized tax benefits, expense (benefit) recognized on a gross basis	1	$ (4)	(3)	$ (2)
Interest and penalties related to unrecognized tax benefits, accrued on a gross basis	$ 39		$ 39		45
Increase (decrease) in effective income tax rate from prior reporting period to current reporting period (as a percent)	1.50%		(0.60%)
Impact of factors that decreased the effective tax rate from prior reporting period to current reporting period (as a percent)	(3.80%)		(4.50%)
Impact of factors that increased the effective tax rate from prior reporting period to current reporting period (as a percent)	5.30%		3.90%
Deferred Tax Assets, Valuation Allowance	$ 39		$ 39		$ 31
Reconciliation of Effective Income Tax Rate
Effective tax rate (as a percent)	29.60%	28.10%	28.20%	28.80%