Capitalized servicing assets - Economic Assumptions Used to Determine Fair Value of Capitalized Servicing Rights and Sensitivity of Value to Changes in Assumptions (Parenthetical) (Detail)	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]
Impact on fair value	10.00%
Impact on fair value	10.00%
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]
Impact on fair value	20.00%
Impact on fair value	20.00%
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]
Impact on fair value	10.00%
Impact on fair value	10.00%