Fair Value of Assets and Liabilities - Quantitative Information about Significant Unobservable Inputs Used for Fair Value Measurements (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Fair Value Inputs [Abstract]
Base market servicing rates (% per annum on unpaid principal balance)(1)	0.50%	0.50%
Minimum
Fair Value Inputs [Abstract]
Base market servicing rates (% per annum on unpaid principal balance)(1)	0.40%
Maximum
Fair Value Inputs [Abstract]
Base market servicing rates (% per annum on unpaid principal balance)(1)	0.75%
Level 3 Inputs | Minimum | Loans, notes and certificates
Fair Value Inputs [Abstract]
Discount rates	3.50%	5.20%
Net cumulative expected loss rates	0.30%	0.30%
Cumulative prepayment rates	21.90%
Level 3 Inputs | Minimum | Servicing asset/liability
Fair Value Inputs [Abstract]
Discount rates	3.50%	5.30%
Net cumulative expected loss rates	0.30%	0.30%
Cumulative prepayment rates	8.40%	16.50%
Base market servicing rates (% per annum on unpaid principal balance)(1)	0.50%	0.50%
Level 3 Inputs | Maximum | Loans, notes and certificates
Fair Value Inputs [Abstract]
Discount rates	17.50%	17.40%
Net cumulative expected loss rates	22.00%	22.00%
Cumulative prepayment rates	38.50%
Level 3 Inputs | Maximum | Servicing asset/liability
Fair Value Inputs [Abstract]
Discount rates	20.50%	23.70%
Net cumulative expected loss rates	21.50%	22.00%
Cumulative prepayment rates	48.70%	26.70%
Base market servicing rates (% per annum on unpaid principal balance)(1)	0.75%	0.70%
Level 3 Inputs | Weighted- Average | Loans, notes and certificates
Fair Value Inputs [Abstract]
Discount rates	8.90%	10.10%
Net cumulative expected loss rates	9.70%	10.00%
Cumulative prepayment rates	27.70%
Level 3 Inputs | Weighted- Average | Servicing asset/liability
Fair Value Inputs [Abstract]
Discount rates	9.60%	10.70%
Net cumulative expected loss rates	8.70%	10.20%
Cumulative prepayment rates	29.00%	20.00%
Base market servicing rates (% per annum on unpaid principal balance)(1)	0.50%	0.50%