Financial Guarantee Insurance - Summary of Percentage Ceded to Reinsurers and Reinsurance Recoverable and Rating Levels (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items]
Percentage ceded Par	100.00%
Net unsecured reinsurance recoverable	$ 0
Assured Guaranty Re Ltd [Member] | Moody's NR Rating [Member]
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items]
Percentage ceded Par	82.90%
Net unsecured reinsurance recoverable	$ 0
Sompo Japan Nipponkoa Holdings, Inc [Member] | Moody's, A1 Rating [Member]
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items]
Percentage ceded Par	7.90%
Net unsecured reinsurance recoverable	$ 0
Assured Guaranty Corporation [Member] | Moody's, A3 Rating [Member]
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items]
Percentage ceded Par	9.20%
Net unsecured reinsurance recoverable	$ 0