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Financial Guarantee Insurance Contracts - Effect of Reinsurance on Premiums Written and Earned (Detail) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]                      
Direct Premiums Written                 $ 23,828 $ 14,313 $ 53,837
Assumed Reinsurance Premiums Written                 0 0 0
Ceded Reinsurance Premiums Written                 (16,860) 2,104 8,772
Premiums written, net of reinsurance                 40,688 12,209 45,065
Direct Premiums Earned                 118,977 190,496 215,564
Premiums Earned, Net                 111,089 175,277 197,287
Assumed Reinsurance Premiums Earned                 79 106 85
Ceded Reinsurance Premiums Earned                 7,967 15,325 18,362
Reinsurance on premiums earned, Net $ 28,730 $ 25,640 $ 25,836 $ 30,883 $ 31,523 $ 52,989 $ 43,152 $ 47,613 $ 111,089 $ 175,277 $ 197,287