Transfers of financial assets and servicing rights - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended					6 Months Ended	12 Months Ended	3 Months Ended		6 Months Ended					3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 Loans With Recourse [Member]	Dec. 31, 2012 Loans With Recourse [Member]	Jun. 30, 2013 Buy Back Option Program [Member]	Dec. 31, 2012 Buy Back Option Program [Member]	Jun. 30, 2013 Residential Mortgage	Jun. 30, 2012 Residential Mortgage	Jun. 30, 2013 Residential Mortgage	Jun. 30, 2012 Residential Mortgage	Jun. 30, 2013 Serviced Mortgage Loans [Member]	Jun. 30, 2012 Serviced Mortgage Loans [Member]	Dec. 31, 2012 Serviced Mortgage Loans [Member]	Jun. 30, 2013 Serviced Mortgage Loans [Member] Loans With Recourse [Member]	Jun. 30, 2012 Serviced Mortgage Loans [Member] Loans With Recourse [Member]	Jun. 30, 2013 Serviced Mortgage Loans [Member] Loans With Recourse [Member]	Jun. 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														$ 40,000,000	$ 118,000,000
Securitization Financial Asset For Which Transfer Is Accounted As Sale Gain Loss On Sale										8,800,000	13,900,000	26,500,000	27,600,000	1,500,000	4,600,000
Loans serviced						2,700,000,000	2,900,000,000							16,600,000,000		16,700,000,000
Mortgage Servicing Fees Excluding Fair Value	11,300,000	11,900,000	22,600,000	24,100,000
Weighted average mortgage servicing fees	0.27%	0.28%	0.27%	0.28%
Mortgage loans related to buy-back option program	21,615,754,000		21,615,754,000		21,080,005,000			53,000,000	56,000,000
Repurchased Loans								$ 56,000,000	$ 255,000,000								$ 36,000,000	$ 32,000,000	$ 66,000,000	$ 82,000,000