Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2017	Dec. 31, 2015
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Unrecognized Tax Benefits	$ 1,056			$ 1,056
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	901			900
Incremental tax liability asserted by IRS	$ 1,162
Reported tax rate	11.40%	97.50%
Net tax expense/benefits related to interest and penalties	$ 10	$ 11
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued, Total	$ 516			$ 500
Subsequent Event [Member]
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Potential Reduction In Unrecognized Tax Benefits Over Next Twelve Months As Result Of Concluding Certain Matters			$ 15