Financial Guarantee Liability - Provisions for Risk Sharing (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Guarantees [Abstract]
Increase (decrease) to financial guarantee liability	$ (14)	$ (208)	$ (7)	$ (213)
Decrease (increase) to credit enhancement asset		140	10	144
Increase (decrease) to contingent liability	1	6	1	6
Total provision for risk-sharing obligations	$ (13)	$ (62)	$ 4	$ (63)