Capitalized servicing assets - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Commercial Mortgage Loans [Member]
Servicing Assets at Fair Value [Line Items]
Mortgage loans serviced for others	$ 10,600,000,000	$ 9,000,000,000	$ 8,100,000,000
Estimated fair value of capitalized mortgage loan servicing assets	71,000,000	60,000,000
Weighted average discount rates used to estimate fair value of capitalized mortgaged loan servicing assets	18.00%	18.00%
Residential Mortgage Loans [Member]
Servicing Assets at Fair Value [Line Items]
Mortgage loans serviced for others	32,100,000,000	36,300,000,000	15,900,000,000
Estimated fair value of capitalized mortgage loan servicing assets	138,000,000	168,000,000
Weighted average discount rates used to estimate fair value of capitalized mortgaged loan servicing assets	10.60%	10.40%
Weighted-average option-adjusted spread in the discount rate	929 basis points	978 basis points
Small-Balance Commercial Mortgage Loans [Member]
Servicing Assets at Fair Value [Line Items]
Mortgage loans serviced for others	3,800,000,000	4,400,000,000	5,200,000,000
Estimated fair value of capitalized mortgage loan servicing assets	$ 43,000,000	$ 49,000,000
Weighted average discount rates used to estimate fair value of capitalized mortgaged loan servicing assets	19.40%	19.20%
Weighted-average option-adjusted spread in the discount rate	1,791 basis points	1,783 basis points