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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) - Successor [Member] - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 Months, Fair Value $ 1,211,802 $ 1,988,778
Less than 12 Months, Gross Unrealized Loss 28,374 86,480
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 321,091 633,662
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 11,671 21,563
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 1,532,893 2,622,440
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 40,045 108,043
Below Investment Grade Securities and Non-Rated Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Fair Value 694,796 1,855,694
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Aggregate Losses 18,330 100,503
Short-Term [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 Months, Fair Value 120,661 251,926
Less than 12 Months, Gross Unrealized Loss 95 209
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 0 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 120,661 251,926
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 95 209
Fixed Income Investments And Other Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 40,045 108,043
Fixed Income Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 Months, Fair Value 1,091,141 1,736,852
Less than 12 Months, Gross Unrealized Loss 28,279 86,271
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 321,091 633,662
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 11,671 21,563
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 1,412,232 2,370,514
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 39,950 107,834
Fixed Income Securities [Member] | Municipal Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 Months, Fair Value 495,224 667,335
Less than 12 Months, Gross Unrealized Loss 10,775 68,578
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 30,590 32,525
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 1,146 822
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 525,814 699,860
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 11,921 69,400
Fixed Income Securities [Member] | Corporate Obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 Months, Fair Value 457,056 292,028
Less than 12 Months, Gross Unrealized Loss 13,566 3,377
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 106,696 87,272
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 5,243 2,094
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 563,752 379,300
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 18,809 5,471
Fixed Income Securities [Member] | U.S. Government Obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 Months, Fair Value 77,446 74,188
Less than 12 Months, Gross Unrealized Loss 3,067 1,653
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 3,423 5,525
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 72 100
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 80,869 79,713
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 3,139 1,753
Fixed Income Securities [Member] | U.S. Agency Obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 Months, Fair Value 0 0
Less than 12 Months, Gross Unrealized Loss 0 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 0 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 0 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 0 0
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 Months, Fair Value 13,321 668,524
Less than 12 Months, Gross Unrealized Loss 531 12,524
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 101,462 418,617
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 4,389 17,958
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 114,783 1,087,141
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 4,920 30,482
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 Months, Fair Value 0 0
Less than 12 Months, Gross Unrealized Loss 0 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 0 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 0 0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 0 0
Fixed Income Securities [Member] | Other Asset-Backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 Months, Fair Value 34,288 26,655
Less than 12 Months, Gross Unrealized Loss 123 58
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 77,184 88,023
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 770 559
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 111,472 114,678
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 893 617
Fixed Income Securities [Member] | Foreign Government Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Less than 12 Months, Fair Value 13,806 8,122
Less than 12 Months, Gross Unrealized Loss 217 81
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value 1,736 1,700
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 51 30
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 15,542 9,822
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 268 111
Securities Pledged as Collateral [Member] | Fixed Income Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 0 0
Securities Pledged as Collateral [Member] | Fixed Income Securities [Member] | U.S. Government Obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments $ 0 $ 0