Concentrations of Credit Risk - Additional Information (Detail) - Fannie Mae DUS and Freddie Mac TAH Loans [Member] - Liabilities [Member] - Credit Concentration Risk [Member] - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2019	Dec. 31, 2018
California [Member]
Concentration Risk [Line Items]
Concentration risk percentage	22.00%	25.00%
Texas [Member]
Concentration Risk [Line Items]
Concentration risk percentage	16.00%	16.00%
California and Texas [Member]
Concentration Risk [Line Items]
Maximum pre-credit enhancement loss exposure	$ 5,700,000,000	$ 5,800,000,000

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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