Transfers of financial assets and servicing rights - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended					3 Months Ended			3 Months Ended		9 Months Ended							3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Loans With Recourse [Member]	Dec. 31, 2011 Loans With Recourse [Member]	Sep. 30, 2011 Loans With Recourse [Member]	Sep. 30, 2012 Buy Back Option Program [Member]	Dec. 31, 2011 Buy Back Option Program [Member]	Sep. 30, 2011 Buy Back Option Program [Member]	Sep. 30, 2012 Residential Mortgage	Sep. 30, 2011 Residential Mortgage	Sep. 30, 2012 Residential Mortgage	Sep. 30, 2011 Residential Mortgage	Sep. 30, 2012 Loan	Sep. 30, 2011 Loan	Sep. 30, 2012 Serviced Mortgage Loans [Member]	Dec. 31, 2011 Serviced Mortgage Loans [Member]	Sep. 30, 2011 Serviced Mortgage Loans [Member]	Sep. 30, 2012 Serviced Mortgage Loans [Member] Loans With Recourse [Member]	Sep. 30, 2011 Serviced Mortgage Loans [Member] Loans With Recourse [Member]	Sep. 30, 2012 Serviced Mortgage Loans [Member] Loans With Recourse [Member]	Sep. 30, 2011 Serviced Mortgage Loans [Member] Loans With Recourse [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															$ 196,000,000	$ 84,000,000
Securitization Financial Asset For Which Transfer Is Accounted As Sale Gain Loss On Sale											18,000,000	1,600,000	45,600,000	12,000,000	8,900,000	1,700,000
Loans serviced					3,100,000,000	3,500,000,000	3,600,000,000										16,800,000,000	17,300,000,000	17,400,000,000
Mortgage Servicing Fees Excluding Fair Value	12,200,000	12,200,000	36,300,000	37,000,000
Weighted average mortgage servicing fees	0.28%	0.27%	0.28%	0.27%
Mortgage loans related to buy-back option program								70,000,000	180,000,000	163,000,000
Repurchased Loans								$ 184,000,000												$ 33,000,000	$ 53,000,000	$ 115,000,000	$ 168,000,000