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	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Concentration Risk [Line Items]
Concentration risk percentage	100.00%	100.00%
California
Concentration Risk [Line Items]
Concentration risk percentage	76.00%	49.00%
Florida
Concentration Risk [Line Items]
Concentration risk percentage	7.00%	43.00%
Texas
Concentration Risk [Line Items]
Concentration risk percentage	3.00%	5.00%
Other states
Concentration Risk [Line Items]
Concentration risk percentage	14.00%	3.00%