Income Tax Provision - Deferred Federal and State Income Tax Liabilities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Deferred Tax Assets Liabilities, Net [Line Items]
Net deferred income tax liability	$ 7,519	[1]	$ 6,747	[1]
Federal [Member]
Deferred Tax Assets Liabilities, Net [Line Items]
Net deferred income tax liability	6,873		6,217
State [Member]
Deferred Tax Assets Liabilities, Net [Line Items]
Net deferred income tax liability	646		529
Foreign [Member]
Deferred Tax Assets Liabilities, Net [Line Items]
Net deferred income tax liability	$ 0		$ 1

[1] Balance includes deferred tax liabilities attributable to unrealized gain or loss included in accumulated other comprehensive income or loss, net of $136 million and $124 million at March 31, 2015 and 2014, respectively. The change in this deferred liability is not included in total deferred tax expense.