Financial Guarantee Liability - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Guarantor Obligations [Line Items]
Outstanding loan balances	$ 20,900,000	$ 20,200,000
Maximum loss potential	6,100,000	5,800,000
Portion covered by Credit Enhancement Agreement	9,700	$ 9,800
Provision for expected credit losses	$ 14,480
Fannie Mae DUS or Freddie TAH Loans | Maximum
Guarantor Obligations [Line Items]
Percentage of contingent liability of actual losses incurred on outstanding loans	33.00%