Investment Securities - Additional Information (Detail) (USD $) In Billions, unless otherwise specified	Mar. 31, 2013 Y	Dec. 31, 2012 Y
Investments Debt And Equity Securities [Abstract]
Weighted-average maturity of the available-for-sale investment securities (in years)	4.4	4.1
Weighted-average yields of available-for-sale investment securities	2.80%	2.93%
Weighted-average maturity of the held-to-maturity investment securities (in years)	3.6	3.3
Weighted-average yields of held-to-maturity investment securities	1.91%	1.94%
Securities pledged	$ 18.1	$ 20.1
Securities pledged as collateral where counterparty has right to repledge or resell	$ 2.5	$ 3.5