Transfers of financial assets and servicing rights - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended					9 Months Ended	12 Months Ended	3 Months Ended		9 Months Ended					3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Loans With Recourse [Member]	Dec. 31, 2012 Loans With Recourse [Member]	Sep. 30, 2013 Buy Back Option Program [Member]	Dec. 31, 2012 Buy Back Option Program [Member]	Sep. 30, 2013 Residential Mortgage	Sep. 30, 2012 Residential Mortgage	Sep. 30, 2013 Residential Mortgage	Sep. 30, 2012 Residential Mortgage	Sep. 30, 2013 Serviced Mortgage Loans [Member]	Sep. 30, 2012 Serviced Mortgage Loans [Member]	Dec. 31, 2012 Serviced Mortgage Loans [Member]	Sep. 30, 2013 Serviced Mortgage Loans [Member] Loans With Recourse [Member]	Sep. 30, 2012 Serviced Mortgage Loans [Member] Loans With Recourse [Member]	Sep. 30, 2013 Serviced Mortgage Loans [Member] Loans With Recourse [Member]	Sep. 30, 2012 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														$ 116,000,000	$ 196,000,000
Securitization Financial Asset For Which Transfer Is Accounted As Sale Gain Loss On Sale										6,500,000	18,000,000	33,000,000	45,600,000	4,000,000	8,900,000
Loans serviced						2,600,000,000	2,900,000,000							17,100,000,000		16,700,000,000
Mortgage Servicing Fees Excluding Fair Value	11,500,000	12,200,000	34,100,000	36,300,000
Weighted average mortgage servicing fees	0.27%	0.28%	0.27%	0.28%
Mortgage loans related to buy-back option program	21,520,054,000		21,520,054,000		21,080,005,000			51,000,000	56,000,000
Repurchased Loans								$ 155,000,000	$ 255,000,000								$ 29,000,000	$ 33,000,000	$ 95,000,000	$ 115,000,000