Fair Value Measurements - Unobservable Inputs Assumptions - Loans Held for Investment (Detail) - Loans Held for Investment at Fair Value - Level 3	Jun. 30, 2018	Dec. 31, 2017
Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Coupon	6.90%	6.90%
FICO	627	627
Loan-to-value (LTV)	88.00%	88.00%
Weighted Average | Single family
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	87.00%	87.00%
Weighted Average | Manufactured housing
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	4.00%	4.00%
Weighted Average | Multi-family/mixed use/other
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	9.00%	9.00%
Weighted Average | Owner Occupied
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	93.00%	95.00%
Weighted Average | Investor
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	1.00%	1.00%
Weighted Average | Secondary
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Weighted average percent	6.00%	4.00%
Base Rate
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Coupon	5.00%	4.80%
FICO	636	636
Loan-to-value (LTV)	88.00%	89.00%