Key Assumptions Used in Calculations of Fair Value of Residential Mortgage Servicing Rights (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Mortgage Loans on Real Estate [Line Items]
Weighted Average Expected Life	56 months	69 months
Constant Prepayment Speed	17.70%	14.20%
Discount Rate	10.50%	10.00%
Primary Mortgage Rate to Refinance	3.50%	4.10%
Current
Mortgage Loans on Real Estate [Line Items]
Cost to service per loan per year	53	53
30-59 days or less delinquent
Mortgage Loans on Real Estate [Line Items]
Cost to service per loan per year	103	103
60-89 days delinquent
Mortgage Loans on Real Estate [Line Items]
Cost to service per loan per year	203	203
90-119 days delinquent
Mortgage Loans on Real Estate [Line Items]
Cost to service per loan per year	303	303
Over 120 days delinquent
Mortgage Loans on Real Estate [Line Items]
Cost to service per loan per year	553	553