Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2018	Mar. 31, 2018	Dec. 31, 2016
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Unrecognized Tax Benefits	$ 1,106				$ 1,095
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	1,017				1,006
Incremental tax liability asserted by IRS	$ 1,162
Reported tax rate	4.90%	11.40%
Net tax expense/benefits related to interest and penalties	$ 13	$ 10
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued, Total	$ 592				$ 572
Subsequent Event [Member]
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Litigation Settlement, Amount			$ 275
Potential Reduction In Unrecognized Tax Benefits Over Next Twelve Months As Result Of Concluding Certain Matters				$ 757