Fair Value Measurements - Unobservable Inputs Assumptions - Loans Held for Investment (Detail) - Loans Held for Investment at Fair Value - Level 3	Dec. 31, 2019	Dec. 31, 2018
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Coupon	6.60%	6.90%
FICO	624	626
Loan-to-value (LTV)	88.00%	88.00%
Single family
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	85.00%	86.00%
Manufactured housing
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	4.00%	4.00%
Multi-family/mixed use/other
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	11.00%	10.00%
Owner Occupied
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	89.00%	90.00%
Investor
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	2.00%	1.00%
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	4.30%	5.20%
FICO	636	638
Loan-to-value (LTV)	86.00%	88.00%