Fair Value Measurements - Unobservable Inputs Assumptions - Loans Held for Investment (Detail) - Loans Held for Investment at Fair Value - Level 3	Mar. 31, 2020	Dec. 31, 2019
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Coupon	6.40%	6.60%
FICO	628	624
Loan-to-value (LTV)	87.00%	88.00%
Single family
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	85.00%	85.00%
Manufactured housing
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	3.00%	4.00%
Multi-family/mixed use/other
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	12.00%	11.00%
Owner Occupied
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	89.00%	89.00%
Investor
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	2.00%	2.00%
Secondary
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	9.00%	9.00%
Base Rate
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Coupon	5.50%	4.30%
FICO	637	636
Loan-to-value (LTV)	89.00%	86.00%