Capitalized servicing assets - Economic Assumptions Used to Determine Fair Value of Capitalized Servicing Rights and Sensitivity of Value to Changes in Assumptions (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012
Residential Mortgage Loans [Member]
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Weighted-average prepayment speeds	19.50%
Impact on fair value of 10% adverse change, prepayment speeds	$ 8,924,000
Impact on fair value of 20% adverse change, prepayment speeds	16,880,000
Weighted-average OAS	9.29%
Impact on fair value of 10% adverse change, OAS	2,857,000
Impact on fair value of 20% adverse change, OAS	5,577,000
Small-Balance Commercial Mortgage Loans [Member]
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Weighted-average prepayment speeds	9.16%
Impact on fair value of 10% adverse change, prepayment speeds	1,814,000
Impact on fair value of 20% adverse change, prepayment speeds	3,480,000
Weighted-average OAS	17.91%
Impact on fair value of 10% adverse change, OAS	1,468,000
Impact on fair value of 20% adverse change, OAS	2,832,000
Commercial Mortgage Loans [Member]
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Weighted-average discount rate	18.00%
Impact on fair value of 10% adverse change, discount rate	(3,078,000)
Impact on fair value of 20% adverse change, discount rate	$ (5,940,000)