Residential Mortgage Loans Held-For-Securitization - Summary of Geographic Concentrations of Residential Mortgage Loans Held-for-Investment (Detail) - Geographic Concentration Risk - Residential Loans Held For Investment	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Concentration Risk [Line Items]
Concentration risk percentage	100.00%	100.00%
California
Concentration Risk [Line Items]
Concentration risk percentage	73.00%	74.00%
Florida
Concentration Risk [Line Items]
Concentration risk percentage	7.00%	7.00%
New York
Concentration Risk [Line Items]
Concentration risk percentage	6.00%	6.00%
Other states
Concentration Risk [Line Items]
Concentration risk percentage	14.00%	13.00%