Financial Guarantee Liability - Additional Information (Detail)	3 Months Ended	9 Months Ended
	Sep. 30, 2020 USD ($) loan	Sep. 30, 2020 USD ($) loan	Dec. 31, 2019 USD ($)
Guarantor Obligations
Outstanding loan balances	$ 22,700,000,000	$ 22,700,000,000	$ 20,200,000,000
Maximum loss potential	6,700,000,000	6,700,000,000	5,800,000,000
Portion covered by Credit Enhancement Agreement	9,600,000	9,600,000	$ 9,800,000
Provision for expected credit losses	0	14,702,000
Delinquent
Guarantor Obligations
Outstanding loan balances	53,500,000	53,500,000
Maximum loss potential	$ 17,800,000	$ 17,800,000
Number of loans (loans) | loan	4	4
Liquidation value of loans outstanding	$ 41,700,000	$ 41,700,000
Potential loss on liquidation of loan		$ 6,000,000.0
Fannie Mae DUS or Freddie TAH Loans | Maximum
Guarantor Obligations
Percentage of contingent liability of actual losses incurred on outstanding loans (in percentage)		33.00%