Concentration of Credit Risk - Additional Information (Detail) - Fannie Mae DUS and Freddie Mac TAH Loans [Member] - Liabilities [Member] - Credit Concentration Risk [Member] - USD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
California [Member]
Concentration Risk [Line Items]
Concentration risk percentage	25.00%	26.00%
Texas [Member]
Concentration Risk [Line Items]
Concentration risk percentage	16.00%	15.00%
California and Texas [Member]
Concentration Risk [Line Items]
Maximum pre-credit enhancement loss exposure	$ 5,800,000,000	$ 5,300,000,000

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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

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