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Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Statements of Stockholders' Equity (Detail) - Successor [Member] - USD ($) | 12 Months Ended | 30 Months Ended |
--- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2017 | Dec. 31, 2014
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
Total comprehensive income (loss) | $ (335,376,000) | $ 20,112,000 | $ 287,252,000 |  |
Stock-based compensation | (4,293,000) | (5,253,000) | (3,105,000) |  |
Cost of shares (acquired) issued under equity plan | (1,547,000) | (505,000) | (374,000) |  |
Payments for Repurchase of Warrants | 0 | 2,717,000 | 5,375,000 | $ 8,092,000 |
Stock Issued During Period, Value, New Issues | 1,000 | 2,000 |  |  |
Noncontrolling Interest, Decrease from Deconsolidation |  | (2,469,000) |  |  |
Proceeds from Warrant Exercises | 0 | 2,000 | 3,000 |  |
Warrants exercised |  | 2,000 | 3,000 |  |
Ending balance | 1,645,258,000 | 1,978,024,000 | 1,958,346,000 | 1,645,258,000 |
Stockholders' Equity Attributable to Parent | 1,381,148,000 | 1,713,914,000 |  | 1,381,148,000 |
Retained Earnings [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
Total comprehensive income (loss) | (328,715,000) | 74,843,000 | 493,403,000 |  |
Stock-based compensation | 0 | 0 | 0 |  |
Cost of shares (acquired) issued under equity plan | (1,572,000) | (127,000) | (312,000) |  |
Payments for Repurchase of Warrants | 0 | 1,916,000 | 3,942,000 |  |
Stock Issued During Period, Value, New Issues | 0 | 0 |  |  |
Noncontrolling Interest, Decrease from Deconsolidation |  | 0 |  |  |
Warrants exercised |  | 0 | 0 |  |
Ending balance | 1,233,845,000 | 1,557,681,000 | 1,478,439,000 | 1,233,845,000 |
AOCI Attributable to Parent [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
Total comprehensive income (loss) | (6,661,000) | (54,205,000) | (205,068,000) |  |
Stock-based compensation | 0 | 0 | 0 |  |
Cost of shares (acquired) issued under equity plan | 0 | 0 | 0 |  |
Payments for Repurchase of Warrants | 0 | 0 | 0 |  |
Stock Issued During Period, Value, New Issues | 0 | 0 |  |  |
Noncontrolling Interest, Decrease from Deconsolidation |  | 0 |  |  |
Warrants exercised |  | 0 | 0 |  |
Ending balance | (52,239,000) | (38,990,000) | 15,215,000 | (52,239,000) |
Preferred Stock [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
Total comprehensive income (loss) | 0 | 0 | 0 |  |
Stock-based compensation | 0 | 0 | 0 |  |
Cost of shares (acquired) issued under equity plan | 0 | 0 | 0 |  |
Payments for Repurchase of Warrants | 0 | 0 | 0 |  |
Stock Issued During Period, Value, New Issues | 0 | 0 |  |  |
Noncontrolling Interest, Decrease from Deconsolidation |  | 0 |  |  |
Warrants exercised |  | 0 | 0 |  |
Ending balance | 0 | 0 | 0 | 0 |
Common Stock [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
Total comprehensive income (loss) | 0 | 0 | 0 |  |
Stock-based compensation | 0 | 0 | 0 |  |
Cost of shares (acquired) issued under equity plan | 0 | 0 | 0 |  |
Payments for Repurchase of Warrants | 0 | 0 | 0 |  |
Stock Issued During Period, Value, New Issues | 1,000 | 2,000 |  |  |
Noncontrolling Interest, Decrease from Deconsolidation |  | 0 |  |  |
Warrants exercised |  | 0 | 0 |  |
Ending balance | 453,000 | 452,000 | 450,000 | 453,000 |
Additional Paid-in Capital [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
Total comprehensive income (loss) | 0 | 0 | 0 |  |
Stock-based compensation | (4,293,000) | (5,253,000) | (3,105,000) |  |
Cost of shares (acquired) issued under equity plan | 0 | 0 | 0 |  |
Payments for Repurchase of Warrants | 0 | 801,000 | 1,433,000 |  |
Stock Issued During Period, Value, New Issues | 0 | 0 |  |  |
Noncontrolling Interest, Decrease from Deconsolidation |  | 0 |  |  |
Warrants exercised |  | 2,000 | 3,000 |  |
Ending balance | 199,560,000 | 195,267,000 | 190,813,000 | 199,560,000 |
Common Stock Held in Treasury, at Cost [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
Total comprehensive income (loss) | 0 | 0 | 0 |  |
Stock-based compensation | 0 | 0 | 0 |  |
Cost of shares (acquired) issued under equity plan | 25,000 | (378,000) | (62,000) |  |
Payments for Repurchase of Warrants | 0 | 0 | 0 |  |
Stock Issued During Period, Value, New Issues | 0 | 0 |  |  |
Noncontrolling Interest, Decrease from Deconsolidation |  | 0 |  |  |
Warrants exercised |  | 0 | 0 |  |
Ending balance | (471,000) | (496,000) | (118,000) | (471,000) |
Noncontrolling Interest [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
Total comprehensive income (loss) | 0 | (526,000) | (1,083,000) |  |
Stock-based compensation | 0 | 0 | 0 |  |
Cost of shares (acquired) issued under equity plan | 0 | 0 | 0 |  |
Payments for Repurchase of Warrants | 0 | 0 | 0 |  |
Stock Issued During Period, Value, New Issues | 0 | 0 |  |  |
Noncontrolling Interest, Decrease from Deconsolidation |  | (2,469,000) |  |  |
Warrants exercised |  | 0 | 0 |  |
Ending balance | 264,110,000 | 264,110,000 | 273,547,000 | 264,110,000 |
Ambac Financial Group, Inc Parent Company Only [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
Total comprehensive income (loss) | (335,376,000) | 20,638,000 | 288,335,000 |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | (137,000) | 6,442,000 |  |  |
Stock-based compensation | (4,293,000) | (5,253,000) | (3,105,000) |  |
Cost of shares (acquired) issued under equity plan | (1,547,000) | (505,000) | (374,000) |  |
Payments for Repurchase of Warrants | 0 | (2,717,000) | (5,375,000) |  |
Stock Issued During Period, Value, New Issues | 1,000 | 2,000 |  |  |
Proceeds from Warrant Exercises | 0 | 2,000 | 3,000 |  |
Warrants exercised |  | 2,000 | 3,000 |  |
Stockholders' Equity Attributable to Parent | 1,381,148,000 | 1,713,914,000 | 1,684,799,000 | 1,381,148,000 | $ 1,399,105,000
Ambac Financial Group, Inc Parent Company Only [Member] | Retained Earnings [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
Total comprehensive income (loss) | (328,715,000) | 74,843,000 | 493,403,000 |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | (137,000) | 6,442,000 |  |  |
Stock-based compensation | 0 | 0 | 0 |  |
Cost of shares (acquired) issued under equity plan | (1,572,000) | (127,000) | (312,000) |  |
Payments for Repurchase of Warrants |  | (1,916,000) | (3,942,000) |  |
Stock Issued During Period, Value, New Issues |  | 0 |  |  |
Warrants exercised |  | 0 | 0 |  |
Stockholders' Equity Attributable to Parent | 1,233,845,000 | 1,557,681,000 | 1,478,439,000 | 1,233,845,000 | 989,290,000
Ambac Financial Group, Inc Parent Company Only [Member] | AOCI Attributable to Parent [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
Total comprehensive income (loss) | (6,661,000) | (54,205,000) | (205,068,000) |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification |  | 0 |  |  |
Stock-based compensation | 0 | 0 | 0 |  |
Cost of shares (acquired) issued under equity plan | 0 | 0 | 0 |  |
Payments for Repurchase of Warrants |  | 0 | 0 |  |
Stock Issued During Period, Value, New Issues |  | 0 |  |  |
Warrants exercised |  | 0 | 0 |  |
Stockholders' Equity Attributable to Parent | (52,239,000) | (38,990,000) | 15,215,000 | (52,239,000) | 220,283,000
Ambac Financial Group, Inc Parent Company Only [Member] | Preferred Stock [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
Total comprehensive income (loss) | 0 | 0 | 0 |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification |  | 0 |  |  |
Stock-based compensation | 0 | 0 | 0 |  |
Cost of shares (acquired) issued under equity plan | 0 | 0 | 0 |  |
Payments for Repurchase of Warrants |  | 0 | 0 |  |
Stock Issued During Period, Value, New Issues |  | 0 |  |  |
Warrants exercised |  | 0 | 0 |  |
Stockholders' Equity Attributable to Parent | 0 | 0 | 0 | 0 | 0
Ambac Financial Group, Inc Parent Company Only [Member] | Common Stock [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
Total comprehensive income (loss) | 0 | 0 | 0 |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification |  | 0 |  |  |
Stock-based compensation | 0 | 0 | 0 |  |
Cost of shares (acquired) issued under equity plan | 0 | 0 | 0 |  |
Payments for Repurchase of Warrants |  | 0 | 0 |  |
Stock Issued During Period, Value, New Issues | 1,000 | 2,000 |  |  |
Warrants exercised |  | 0 | 0 |  |
Stockholders' Equity Attributable to Parent | 453,000 | 452,000 | 450,000 | 453,000 | 450,000
Ambac Financial Group, Inc Parent Company Only [Member] | Additional Paid-in Capital [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
Total comprehensive income (loss) | 0 | 0 | 0 |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification |  | 0 |  |  |
Stock-based compensation | (4,293,000) | (5,253,000) | (3,105,000) |  |
Cost of shares (acquired) issued under equity plan | 0 | 0 | 0 |  |
Payments for Repurchase of Warrants |  | (801,000) | (1,433,000) |  |
Stock Issued During Period, Value, New Issues |  | 0 |  |  |
Warrants exercised |  | 2,000 | 3,000 |  |
Stockholders' Equity Attributable to Parent | 199,560,000 | 195,267,000 | 190,813,000 | 199,560,000 | 189,138,000
Ambac Financial Group, Inc Parent Company Only [Member] | Common Stock Held in Treasury, at Cost [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
Total comprehensive income (loss) | 0 | 0 | 0 |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification |  | 0 |  |  |
Stock-based compensation | 0 | 0 | 0 |  |
Cost of shares (acquired) issued under equity plan | 25,000 | (378,000) | (62,000) |  |
Payments for Repurchase of Warrants |  | 0 | 0 |  |
Stock Issued During Period, Value, New Issues |  | 0 |  |  |
Warrants exercised |  | 0 | 0 |  |
Stockholders' Equity Attributable to Parent | (471,000) | $ (496,000) | $ (118,000) | $ (471,000) | $ (56,000)
Accounting Standards Update 2018-02 [Domain] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | 0 |  |  |  |
Accounting Standards Update 2018-02 [Domain] | Retained Earnings [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | 6,588,000 |  |  |  |
Accounting Standards Update 2018-02 [Domain] | AOCI Attributable to Parent [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | (6,588,000) |  |  |  |
Accounting Standards Update 2018-02 [Domain] | Ambac Financial Group, Inc Parent Company Only [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | 0 |  |  |  |
Accounting Standards Update 2018-02 [Domain] | Ambac Financial Group, Inc Parent Company Only [Member] | Retained Earnings [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | 6,588,000 |  |  |  |
Accounting Standards Update 2018-02 [Domain] | Ambac Financial Group, Inc Parent Company Only [Member] | AOCI Attributable to Parent [Member] |  |  |  |  |
Increase (Decrease) in Stockholders' Equity [Roll Forward] |  |  |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | $ (6,588,000) |  |  |  |

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