Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) (Successor [Member], USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 1,133,155	$ 1,324,489
Less than 12 Months, Gross Unrealized Loss	22,671	20,081
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	239,992	350,655
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	6,692	12,188
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,373,147	1,675,144
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	29,363	32,269
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value		77,788
Less than 12 Months, Gross Unrealized Loss		1,244
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		135,076
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments		5,752
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value		453,504
Less than 12 Months, Gross Unrealized Loss		4,998
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		172,045
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments		4,418
Foreign Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value		20,827
Less than 12 Months, Gross Unrealized Loss		748
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		14,277
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments		596
U.S. Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value		7,223
Less than 12 Months, Gross Unrealized Loss		154
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		14,735
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments		273
U.S. Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value		25,039
Less than 12 Months, Gross Unrealized Loss		2
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		4,378
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments		36
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value		413,203
Less than 12 Months, Gross Unrealized Loss		12,391
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		10,076
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments		1,109
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value		5,012
Less than 12 Months, Gross Unrealized Loss		274
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments		0
Other Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value		248,823
Less than 12 Months, Gross Unrealized Loss		155
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		68
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments		4
Short-Term [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	0	8,803
Less than 12 Months, Gross Unrealized Loss	0	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	0	8,803
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	4
Fixed Income Investments And Other Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	1,133,155	1,260,222
Less than 12 Months, Gross Unrealized Loss	22,671	19,970
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	239,992	350,655
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	6,692	12,188
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,373,147	1,610,877
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	29,363	32,158
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	1,133,155	1,251,419
Less than 12 Months, Gross Unrealized Loss	22,671	19,966
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	239,992	350,655
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	6,692	12,188
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,373,147	1,602,074
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	29,363	32,154
Fixed Income Securities [Member] | Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	46,461
Less than 12 Months, Gross Unrealized Loss	412
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	119,412
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	3,792
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	165,873	212,864
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	4,204	6,996
Fixed Income Securities [Member] | Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	278,044
Less than 12 Months, Gross Unrealized Loss	2,081
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	81,739
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	1,209
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	359,783	625,549
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	3,290	9,416
Fixed Income Securities [Member] | Foreign Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	32,206
Less than 12 Months, Gross Unrealized Loss	1,864
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	8,900
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	611
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	41,106	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	1,661
Less than 12 Months, Gross Unrealized Loss	2
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	10,814
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	102
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	12,475	21,958
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	104	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
Less than 12 Months, Gross Unrealized Loss	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	4,359
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	11
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	4,359	29,417
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	11	38
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	547,706
Less than 12 Months, Gross Unrealized Loss	17,336
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	10,266
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	821
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	557,972	423,279
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	18,157	13,500
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	63,834
Less than 12 Months, Gross Unrealized Loss	289
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	4,434
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	144
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	68,268	5,012
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	433	274
Fixed Income Securities [Member] | Other Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	163,243
Less than 12 Months, Gross Unrealized Loss	687
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	68
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	2
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	163,311	248,891
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	689	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	2,475	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