Fair Value Measurements - Additional Information (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2018 Loan $ / Loan	Dec. 31, 2017 $ / Loan
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan segregated, number of pools | Loan	12
Service costs assumed, per loan | $ / Loan	6.50	6.50
Average PSA assumed rate	128.00%	150.00%
Fair Value, Measurements, Recurring | 100% PSA | Fair Value, Inputs, Level 3 | Between First Month and Month 30
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Prepayment rate increase, each month	0.20%
Fair Value, Measurements, Recurring | Prepayment Rate | 100% PSA | Fair Value, Inputs, Level 3 | First Month
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Prepayment rate	0
Fair Value, Measurements, Recurring | Prepayment Rate | 100% PSA | Fair Value, Inputs, Level 3 | Month 30
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Prepayment rate	6.0
Fair Value, Measurements, Recurring | Prepayment Rate | 100% PSA | Fair Value, Inputs, Level 3 | Thereafter
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Prepayment rate	6.0
Fair Value, Measurements, Recurring | Discount Rate | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discount rate	12.5	13.0
Mortgage Servicing Rights
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of loan portfolio	2