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Long-Term Debt (Details) - USD ($) $ in Thousands | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Jul. 31, 2015 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | May 01, 2013 | May 19, 2011
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Secured Debt |  | $ 73,993 |  |  |  | $ 102,403 |  |  |  | $ 73,993 | $ 102,403 |  |  |
Junior surplus note of Ambac Assurance Segregated Account |  |  |  |  |  |  |  |  |  |  |  |  | $ 350,000 |
Secured Debt [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Face Amount |  | $ 73,993 |  |  |  | 102,986 |  |  |  | $ 73,993 | 102,986 |  |  |
5.1% Surplus Notes, General Account, Due 2020 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  | 10.50% |  |  |  |  |  |  |  | 10.50% |  |  |  |
Debt Instrument, Repurchased Face Amount |  | $ 108,134 |  |  |  | 11,804 |  |  |  | $ 108,134 | 11,804 |  |  |
Net realized gains (losses) on extinguishment of debt |  |  |  |  |  |  |  |  |  | 3,603 | 1,677 |  |  |
Debt Instrument, Face Amount |  | $ 754,811 |  |  |  | 862,945 |  |  |  | $ 754,811 | 862,945 |  |  |
5.1% Surplus Notes, Segregated Account, Due 2020 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  | 10.50% |  |  |  |  |  |  |  | 10.50% |  |  |  |
Debt Instrument, Repurchased Face Amount |  | $ 39,102 |  |  |  |  |  |  |  | $ 39,102 |  |  |  |
Net realized gains (losses) on extinguishment of debt |  |  |  |  |  |  |  |  |  | 212 |  |  |  |
Debt Instrument, Face Amount |  | 0 |  |  |  | $ 39,102 |  |  |  | 0 | $ 39,102 |  |  |
5.1% Junior Surplus Notes, Segregated Account Due 2020 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  |  |  |  |  | 19.50% |  |  |  |  | 19.50% |  |  |
Debt Instrument, Face Amount |  | 370,237 |  |  |  | $ 374,036 |  |  |  | 370,237 | $ 374,036 |  |  |
One State Street [Member] | 5.1% Junior Surplus Notes, Segregated Account Due 2020 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument Principal Amount Subject To Repurchase |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 13,056
Debt Instrument, Face Amount |  | 20,237 |  |  |  | $ 24,037 |  |  |  | 20,237 | 24,037 |  |  |
Extinguishment of Debt, Amount |  |  |  |  |  |  |  |  |  | 3,799 | $ 4,002 |  |  |
Variable Interest Entity, Not Primary Beneficiary, Aggregated Disclosure [Member] | 5.1% Junior Surplus Notes, Segregated Account Due 2020 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  |  |  |  |  | 8.40% |  |  |  |  | 8.40% |  |  |
Successor [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  | 991,696 |  |  |  | $ 1,114,405 |  |  |  | 991,696 | $ 1,114,405 |  |  |
Net realized gains (losses) on extinguishment of debt |  | 0 | $ 0 | $ 2,179 | $ 2,741 | 0 | $ 24 | $ 3,586 | $ 1,235 | 4,920 | 4,845 | $ 81 |  |
Successor [Member] | Ambac Assurance Corporation [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  | 991,696 |  |  |  | 1,114,405 |  |  |  | 991,696 | 1,114,405 |  |  |
Secured Debt |  | 73,993 |  |  |  | 102,403 |  |  |  | 73,993 | 102,403 |  |  |
Successor [Member] | Ambac Assurance Corporation [Member] | 5.1% Surplus Notes, General Account, Due 2020 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  | 668,667 |  |  |  | 730,648 |  |  |  | 668,667 | 730,648 |  |  |
Successor [Member] | Ambac Assurance Corporation [Member] | 5.1% Surplus Notes, Segregated Account, Due 2020 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  | 0 |  |  |  | 33,107 |  |  |  | 0 | 33,107 |  |  |
Successor [Member] | Ambac Assurance Corporation [Member] | 5.1% Junior Surplus Notes, Segregated Account Due 2020 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  | 249,036 |  |  |  | $ 248,247 |  |  |  | 249,036 | $ 248,247 |  |  |
Successor [Member] | Variable Interest Entities [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  |  |  |  |  | 6.65% |  |  |  |  | 6.65% |  |  |
Long-term debt |  | $ 12,160,544 |  |  |  | $ 11,155,936 |  |  |  | $ 12,160,544 | $ 11,155,936 |  |  |
Successor [Member] | 5.1% Surplus Notes, General Account, Due 2020 [Member] | Ambac Assurance Corporation [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  | 5.10% |  |  |  | 5.10% |  |  |  | 5.10% | 5.10% |  |  |
Debt Instruments Maturity Year |  |  |  |  |  |  |  |  |  | 2020 | 2020 |  |  |
Successor [Member] | 5.1% Surplus Notes, Segregated Account, Due 2020 [Member] | Ambac Assurance Corporation [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  | 5.10% |  |  |  | 5.10% |  |  |  | 5.10% | 5.10% |  |  |
Debt Instruments Maturity Year |  |  |  |  |  |  |  |  |  | 2020 | 2020 |  |  |
Successor [Member] | 5.1% Junior Surplus Notes, Segregated Account Due 2020 [Member] | Ambac Assurance Corporation [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Stated Percentage |  | 5.10% |  |  |  | 5.10% |  |  |  | 5.10% | 5.10% |  |  |
Debt Instruments Maturity Year |  |  |  |  |  |  |  |  |  | 2020 | 2020 |  |  |
Consolidated Variable Interest Entities [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unpaid Principal Amount Of Fixed Rate Debt Accounted For Under Fair Value Option |  | $ 9,387,884 |  |  |  | $ 8,854,530 |  |  |  | $ 9,387,884 | $ 8,854,530 |  |  |
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months |  | 141,327 |  |  |  |  |  |  |  | 141,327 |  |  |  |
Long-term Debt, Maturities, Repayments of Principal in Year Two |  | 307,915 |  |  |  |  |  |  |  | 307,915 |  |  |  |
Long-term Debt, Maturities, Repayments of Principal in Year Three |  | 44,100 |  |  |  |  |  |  |  | 44,100 |  |  |  |
Long-term Debt, Maturities, Repayments of Principal in Year Four |  | 94,024 |  |  |  |  |  |  |  | 94,024 |  |  |  |
Long-term Debt, Maturities, Repayments of Principal in Year Five |  | $ 0 |  |  |  |  |  |  |  | $ 0 |  |  |  |
Consolidated Variable Interest Entities [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  | 0.96% |  |  |  | 0.82% |  |  |  | 0.96% | 0.82% |  |  |
Consolidated Variable Interest Entities [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  | 8.35% |  |  |  | 13.00% |  |  |  | 8.35% | 13.00% |  |  |
London Interbank Offered Rate (LIBOR) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest Rate on Securities Issued by Delaware Trust | 2.80% |  |  |  |  |  |  |  |  |  |  |  |  |

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