Transfers of financial assets and servicing rights - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended								3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 Loans With Recourse [Member]	Dec. 31, 2011 Loans With Recourse [Member]	Jun. 30, 2011 Loans With Recourse [Member]	Jun. 30, 2012 Buy Back Option Program [Member]	Dec. 31, 2011 Buy Back Option Program [Member]	Jun. 30, 2011 Buy Back Option Program [Member]	Jun. 30, 2012 Residential Mortgage	Jun. 30, 2011 Residential Mortgage	Jun. 30, 2012 Residential Mortgage	Jun. 30, 2011 Residential Mortgage	Jun. 30, 2012 Loans	Jun. 30, 2011 Loans	Jun. 30, 2012 Serviced Mortgage Loans [Member]	Dec. 31, 2011 Serviced Mortgage Loans [Member]	Jun. 30, 2011 Serviced Mortgage Loans [Member]
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Servicing rights on whole loan sales, principal balance															$ 118,000,000	$ 53,000,000
Securitization Financial Asset For Which Transfer Is Accounted As Sale Gain Loss On Sale											13,900,000	4,100,000	27,600,000	10,400,000	4,600,000	1,100,000
Loans serviced					3,200,000,000	3,500,000,000	3,700,000,000										17,000,000,000	17,300,000,000	17,400,000,000
Mortgage Servicing Fees Excluding Fair Value	11,900,000	12,400,000	24,100,000	24,800,000
Weighted average mortgage servicing fees	0.28%	0.26%	0.28%	0.26%
Mortgage loans related to buy-back option program								$ 180,000,000	$ 180,000,000	$ 156,000,000