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Derivative Instruments - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]          
Value of right to reclaim cash collateral and posted margin, recorded in Other assets $ 50,310,000   $ 50,310,000   $ 102,904,000
Value of obligation to return cash collateral, recorded in "Other liabilities" 0   0    
Gains in change in fair value of the call options (9,890,000) $ 17,583,000 (61,461,000) $ 51,706,000  
Net liability fair value of all derivative instruments linked to Ambac's own credit risk 102,147,000   102,147,000   67,071,000
Fair value of posted assets as collateral $ 120,733,000   $ 120,733,000   $ 83,700,000