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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	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 1,954,118	$ 1,324,489
Less than 12 Months, Gross Unrealized Loss	58,351	20,081
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	341,210	350,655
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	12,830	12,188
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,295,328	1,675,144
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	71,181	32,269
Short-Term [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	18,822	8,803
Less than 12 Months, Gross Unrealized Loss	4	4
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 Aggregate Losses Accumulated In Investments	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	18,822	8,803
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	4	4
Fixed Income Investments And Other Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	1,954,118	1,260,222
Less than 12 Months, Gross Unrealized Loss	58,351	19,970
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	341,210	350,655
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	12,830	12,188
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,295,328	1,610,877
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	71,181	32,158
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	1,935,296	1,251,419
Less than 12 Months, Gross Unrealized Loss	58,347	19,966
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	341,210	350,655
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	12,830	12,188
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,276,506	1,602,074
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	71,177	32,154
Fixed Income Securities [Member] | Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	91,214	77,788
Less than 12 Months, Gross Unrealized Loss	1,751	1,244
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	124,358	135,076
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	4,930	5,752
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	215,572	212,864
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	6,681	6,996
Fixed Income Securities [Member] | Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	677,529	453,504
Less than 12 Months, Gross Unrealized Loss	15,731	4,998
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	104,242	172,045
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	1,887	4,418
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	781,771	625,549
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	17,618	9,416
Fixed Income Securities [Member] | Foreign Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	33,646	20,827
Less than 12 Months, Gross Unrealized Loss	1,389	748
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	7,842	14,277
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	893	596
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	41,488	35,104
Fixed Income Securities [Member] | U.S. Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	4,671	7,223
Less than 12 Months, Gross Unrealized Loss	150	154
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	10,789	14,735
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	69	273
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	15,460	21,958
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	219	427
Fixed Income Securities [Member] | U.S. Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	0	25,039
Less than 12 Months, Gross Unrealized Loss	0	2
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	4,281	4,378
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	2	36
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	4,281	29,417
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	2	38
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	626,063	413,203
Less than 12 Months, Gross Unrealized Loss	37,165	12,391
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	73,204	10,076
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	5,020	1,109
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	699,267	423,279
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	42,185	13,500
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	73,563	5,012
Less than 12 Months, Gross Unrealized Loss	876	274
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 Aggregate Losses Accumulated In Investments	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	73,563	5,012
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	876	274
Fixed Income Securities [Member] | Other Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	428,610	248,823
Less than 12 Months, Gross Unrealized Loss	1,285	155
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	16,494	68
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	29	4
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	445,104	248,891
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1,314	159
Fixed Income Securities [Member] | Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	2,282	1,344
Securities Pledged as Collateral [Member] | Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	0	64,267
Less than 12 Months, Gross Unrealized Loss	0	111
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 Aggregate Losses Accumulated In Investments	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	0	64,267
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	111
Securities Pledged as Collateral [Member] | Fixed Income Securities [Member] | U.S. Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	0	64,267
Less than 12 Months, Gross Unrealized Loss	0	111
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	0	64,267
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	111
Securities Pledged as Collateral [Member] | Securities Pledged as Collateral [Member] | U.S. Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 0	$ 0

X

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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