Loans - High LTV Interest-only and Arm Loans (Detail) (USD $) In Billions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Loans [Line Items]
Residential mortgage loans with high LTV and no mortgage insurance	$ 0.9	$ 1.1
Interest-only residential mortgage loans	4.0	3.9
ARM loans	$ 10.2	$ 9.9