Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Ambac Wrapped Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 32,184
Successor [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	1,329,878	$ 1,491,493
Less than 12 Months, Gross Unrealized Loss	21,985	29,944
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	580,790	788,764
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	40,123	49,698
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,910,668	2,280,257
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	62,108	79,642
Successor [Member] | 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	997,251	890,952
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Aggregate Losses	49,242	53,273
Successor [Member] | Short-Term [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	154,834	65,176
Less than 12 Months, Gross Unrealized Loss	101	44
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	154,834	65,176
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	101	44
Fixed Income Investments And Other Investments [Member] | Successor [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	1,264,950
Less than 12 Months, Gross Unrealized Loss	21,969
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	580,790
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	40,123
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,845,740
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	62,092	79,642
Fixed Income Securities [Member] | Successor [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	1,110,116	1,426,317
Less than 12 Months, Gross Unrealized Loss	21,868	29,900
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	580,790	788,764
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	40,123	49,698
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,690,906	2,215,081
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	61,991	79,598
Fixed Income Securities [Member] | Successor [Member] | Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	226,184	98,147
Less than 12 Months, Gross Unrealized Loss	7,956	2,045
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	66,681	122,928
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	3,976	5,160
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	292,865	221,075
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	11,932	7,205
Fixed Income Securities [Member] | Successor [Member] | Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	507,877	963,513
Less than 12 Months, Gross Unrealized Loss	9,029	20,232
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,159	6,492
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	15	328
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	509,036	970,005
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	9,044	20,560
Fixed Income Securities [Member] | Successor [Member] | U.S. Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	32,710	6,037
Less than 12 Months, Gross Unrealized Loss	762	93
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	5,045
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	4
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	32,710	11,082
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	762	97
Fixed Income Securities [Member] | Successor [Member] | U.S. Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	0	4,060
Less than 12 Months, Gross Unrealized Loss	0	3
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	4,060
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	3
Fixed Income Securities [Member] | Successor [Member] | Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	254,070	226,889
Less than 12 Months, Gross Unrealized Loss	3,895	7,201
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	434,077	550,807
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	28,289	36,584
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	688,147	777,696
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	32,184	43,785
Fixed Income Securities [Member] | Successor [Member] | Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	0	6,986
Less than 12 Months, Gross Unrealized Loss	0	23
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	8,005	25,780
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2	197
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	8,005	32,766
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	2	220
Fixed Income Securities [Member] | Successor [Member] | Other Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	79,459	115,622
Less than 12 Months, Gross Unrealized Loss	132	203
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	70,868	77,712
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	7,841	7,425
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	150,327	193,334
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	7,973	7,628
Fixed Income Securities [Member] | Successor [Member] | Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	9,816	5,063
Less than 12 Months, Gross Unrealized Loss	94	100
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	9,816	5,063
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	94	100
Securities Pledged as Collateral [Member] | Fixed Income Securities [Member] | Successor [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	64,928
Less than 12 Months, Gross Unrealized Loss	16
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, Accumulated Loss	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	64,928
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	16	0
Securities Pledged as Collateral [Member] | Fixed Income Securities [Member] | Successor [Member] | U.S. Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	64,928
Less than 12 Months, Gross Unrealized Loss	16
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, Accumulated Loss	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	64,928
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 16	$ 0