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Variable Interest Entities - Summary of Carrying Amount of Assets, Liabilities and Maximum Exposure to Loss of Ambac's Variable Interests in Non-Consolidated Variable Interest Entities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 35,566,155	$ 44,199,767
Insurance Assets	2,257,824	1,069,268
Insurance Liabilities	1,433,877	4,260,578
Derivative Liabilities	5,711
Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	11,420,199	18,569,951
Insurance Assets	1,948,753	733,921
Insurance Liabilities	1,098,440	3,889,522
Derivative Liabilities	7,168	10,296
Global Structured Finance [Member] | Collateralized Debt Obligations [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	9,787	35,555
Insurance Assets	0	169
Insurance Liabilities	0	1
Derivative Liabilities	(2)
Global Structured Finance [Member] | Residential Mortgage-Backed Securities [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	6,713,437	12,766,685
Insurance Assets	1,859,121	619,848
Insurance Liabilities	546,682	3,218,356
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Consumer Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	1,700,984	2,266,610
Insurance Assets	15,435	23,405
Insurance Liabilities	238,234	328,732
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Commercial Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	873,343	987,797
Insurance Assets	20,735	30,413
Insurance Liabilities	12,264	35,976
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Debt Obligations [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	2,122,648	2,513,304
Insurance Assets	53,462	60,086
Insurance Liabilities	301,260	306,457
Derivative Liabilities	7,170	10,311
Global Public Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	24,145,956	25,629,816
Insurance Assets	309,071	335,347
Insurance Liabilities	335,437	371,056
Derivative Liabilities	$ (1,457)	$ (551)

X

- Definition

The net carrying amount of derivative assets or liabilities in the reporting entity's statement of financial position that relate to the entity's variable interests in Variable Interest Entities (VIEs) where the VIEs are not consolidated.

+ References

No definition available.

+ Details

Name:	ambc_VariableInterestEntityNonconsolidatedCarryingAmountDerivativeNetAssetsOrLiabilities
Namespace Prefix:	ambc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

The carrying amount of insurance assets in the reporting entity's statement of financial position that relate to the entity's variable interests in Variable Interest Entities (VIEs) where the VIEs are not consolidated.

+ References

No definition available.

+ Details

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Namespace Prefix:	ambc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

The carrying amount of insurance liabilities in the reporting entity's statement of financial position that relate to the entity's variable interests in Variable Interest Entities (VIEs) where the VIEs are not consolidated.

+ References

No definition available.

+ Details

Name:	ambc_VariableInterestEntityNonconsolidatedCarryingAmountInsuranceLiabilities
Namespace Prefix:	ambc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=108788376&loc=d3e5747-111685

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph b
-URI http://asc.fasb.org/extlink&oid=108788376&loc=d3e5728-111685

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	credit
Period Type:	instant

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Period Type:	duration

X

- Details

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- Details

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Balance Type:
Period Type:

X

- Details

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X

- Details

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- Details

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- Details

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