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Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Parenthetical) (Detail) (Ambac UK [Member], USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Ambac UK [Member]
  
Investment [Line Items]  
Asset-backed securities fair value $ 51,395ambc_AssetBackedSecuritiesFairValueDisclosure
/ dei_LegalEntityAxis
= ambc_AmbacUkMember