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Fair Value Measurements - Information about Valuation Inputs for Fixed Income Securities Classified as Level 3 (Detail) | 1 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Apr. 30, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2014
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |
Coupon rate | 5.01%ambc_FairValueInputsCouponRate |  |  |
Successor [Member] | United States Transactions [Member] |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |
Coupon rate |  | 5.88%ambc_FairValueInputsCouponRate / us-gaap_FairValueByAssetClassAxis = ambc_UnitedStatesTransactionsMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 6.11%ambc_FairValueInputsCouponRate / us-gaap_FairValueByAssetClassAxis = ambc_UnitedStatesTransactionsMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember |
Fair Value Inputs Maturity |  | 15 years 10 months 4 days | 6 years 9 months 28 days |
Yield |  |  | 11.95%ambc_FairValueInputsYield / us-gaap_FairValueByAssetClassAxis = ambc_UnitedStatesTransactionsMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 7.37%ambc_FairValueInputsYield / us-gaap_FairValueByAssetClassAxis = ambc_UnitedStatesTransactionsMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember
Successor [Member] | European Transactions [Member] |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |
Coupon rate |  | 0.30%ambc_FairValueInputsCouponRate / us-gaap_FairValueByAssetClassAxis = ambc_EuropeanTransactionsMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 0.72%ambc_FairValueInputsCouponRate / us-gaap_FairValueByAssetClassAxis = ambc_EuropeanTransactionsMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember |
Fair Value Inputs Maturity |  | 21 years 2 months 30 days | 17 years 9 months 40 days |
Yield |  | 9.52%ambc_FairValueInputsYield / us-gaap_FairValueByAssetClassAxis = ambc_EuropeanTransactionsMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 8.02%ambc_FairValueInputsYield / us-gaap_FairValueByAssetClassAxis = ambc_EuropeanTransactionsMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember |
Successor [Member] | Corporate Obligations [Member] |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |
Coupon rate |  | 0.345%ambc_FairValueInputsCouponRate / us-gaap_FairValueByAssetClassAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 0.345%ambc_FairValueInputsCouponRate / us-gaap_FairValueByAssetClassAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember |
Fair Value Inputs Maturity |  | 19 years 1 month 21 days | 20 years 1 month 19 days |
Yield |  | 4.93%ambc_FairValueInputsYield / us-gaap_FairValueByAssetClassAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 6.30%ambc_FairValueInputsYield / us-gaap_FairValueByAssetClassAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember |
Successor [Member] | U.S. Agency Obligations [Member] |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |
Coupon rate |  |  | 6.98%ambc_FairValueInputsCouponRate / us-gaap_FairValueByAssetClassAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember |
Fair Value Inputs Maturity |  |  | 1 year 4 months 13 days |
Yield |  |  | 0.39%ambc_FairValueInputsYield / us-gaap_FairValueByAssetClassAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember |
Successor [Member] | Asset-backed Securities [Member] |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |
Coupon rate |  |  | 0.64%ambc_FairValueInputsCouponRate / us-gaap_FairValueByAssetClassAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember |
Fair Value Inputs Maturity |  |  | 7 years 1 month 24 days |
Yield |  |  | 5.17%ambc_FairValueInputsYield / us-gaap_FairValueByAssetClassAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember |

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