Fair Value Fair Value Measurement (Narratives) (Details) (USD $) In Billions, unless otherwise specified	Sep. 30, 2012	Mar. 31, 2012
HARP loans [Member]
Changes due to new accounting guidance or enhanced fair value estimation process [Line Items]
Loans in portfolio that have been refinanced under HARP	$ 199.9
HARP loans [Member] | Fair Value Estimation Process [Member]
Changes due to new accounting guidance or enhanced fair value estimation process [Line Items]
Changes in estimated fair value	5.8	7.4
Fair Value Measurement (Topic 820) Amendment [Member] | Mortgage Loans [Member]
Changes due to new accounting guidance or enhanced fair value estimation process [Line Items]
Changes in estimated fair value		(24.4)
Mortgage Loans [Member]
Changes due to new accounting guidance or enhanced fair value estimation process [Line Items]
Amount of change in fair value of certain loans resulting from updating assumption	$ 23