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Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Balance Sheet (Parenthetical) (Detail) (USD $)
Dec. 31, 2012
Dec. 31, 2011
Condensed Financial Statements, Captions [Line Items]    
Preferred stock, par value $ 0.01 $ 0.01
Preferred stock, Authorized shares 4,000,000 4,000,000
Preferred stock, shares issued 0 0
Preferred stock, shares outstanding 0 0
Common stock, par value $ 0.01 $ 0.01
Common stock, shares authorized shares 650,000,000 650,000,000
Common stock, shares issued 308,016,764 308,016,764
Common stock, shares outstanding 308,016,764 308,016,764
Treasure stock, shares 5,580,657 5,587,953
AMBAC FINANCIAL GROUP, INC PARENT COMPANY ONLY [Member]
    
Condensed Financial Statements, Captions [Line Items]    
Preferred stock, par value $ 0.01 $ 0.01
Preferred stock, Authorized shares 4,000,000 4,000,000
Preferred stock, shares issued 0 0
Preferred stock, shares outstanding 0 0
Common stock, par value $ 0.01 $ 0.01
Common stock, shares authorized shares 650,000,000 650,000,000
Common stock, shares issued 308,016,764 308,016,764
Common stock, shares outstanding 308,016,764 308,016,764
Treasure stock, shares 5,580,657 5,587,953