Fair Value Fair Value Measurement (Narratives) (Details) (USD $) In Billions, unless otherwise specified	Sep. 30, 2012 Mortgage Loans [Member]	Dec. 31, 2012 HARP loans [Member]	Dec. 31, 2012 HARP loans [Member] Fair Value Estimation Process [Member]	Mar. 31, 2012 HARP loans [Member] Fair Value Estimation Process [Member]	Mar. 31, 2012 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			$ 7.6	$ 7.4	$ (24.4)
Loans in portfolio that have been refinanced under HARP		255.2
Amount of change in fair value of certain loans resulting from updating assumption	$ 23