Concentration of Credit Risk - Additional Information (Detail) - Credit Concentration Risk [Member] - Fannie Mae DUS and Freddie Mac TAH Loans [Member] - USD ($) $ in Billions	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
California [Member]
Concentration Risk [Line Items]
Concentration risk percentage	26.00%	26.00%
Texas [Member]
Concentration Risk [Line Items]
Concentration risk percentage	15.00%	15.00%
California and Texas [Member]
Concentration Risk [Line Items]
Maximum amount of loss due to credit risk	$ 5.4	$ 5.3

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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