Fair Value Measurements - Unobservable Inputs Assumptions - Securitized Loans Held for Investment (Detail) - Securitized Loans Held for Investment at Fair Value - Level 3 - point	Mar. 31, 2017	Dec. 31, 2016
Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Coupon	7.10%	7.10%
FICO	637	621
Loan-to-value (LTV)	88.00%	88.00%
Weighted Average | Single family
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	86.00%	86.00%
Weighted Average | Manufactured housing
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	6.00%	6.00%
Weighted Average | Multi-family/mixed use/other
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	8.00%	8.00%
Weighted Average | Owner Occupied
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	97.00%	97.00%
Weighted Average | Investor
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	2.00%	2.00%
Weighted Average | Secondary
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	1.00%	1.00%
Base Rate
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Coupon	5.20%	5.20%
FICO	634	632
Loan-to-value (LTV)	86.00%	87.00%