Financial Guarantees in Force - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Guarantor Obligations [Line Items]
Gross par amount of financial guarantees outstanding	$ 67,140,000	$ 86,373,000
Net par amount of financial guarantees	62,716,000	79,346,000
Gross financial guarantees in force	108,550,000	137,745,000
Net financial guarantees in force	$ 101,223,000	126,306,000
Highest single insured risk of aggregate net par amount guaranteed	2.60%
California [Member]
Guarantor Obligations [Line Items]
Aggregate net par amounts in force	10.10%
New York [Member]
Guarantor Obligations [Line Items]
Aggregate net par amounts in force	5.80%
NEW JERSEY
Guarantor Obligations [Line Items]
Aggregate net par amounts in force	5.20%
No other state [Member] | Maximum [Member]
Guarantor Obligations [Line Items]
Aggregate net par amounts in force	5.00%
Successor [Member]
Guarantor Obligations [Line Items]
Financial Guarantee Insurance Contracts, Accelerated Premium Revenue, Amount	$ 64,494	$ 52,416	$ 137,400