Financial Guarantee Liability - Summary of Provisions for Risk Sharing (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Guarantees [Abstract]
Increase (decrease) to financial guarantee liability		$ (28)	$ 4	$ 13	$ (3)
Decrease (increase) to credit enhancement asset	[1]				10
Increase (decrease) to contingent liability			(1)
Total		$ (28)	$ 3	$ 13	$ 7

[1] The credit enhancement receivable is included in “Other assets” on the accompanying consolidated balance sheets.