Document and Entity Information - USD ($)
12 Months Ended
Dec. 31, 2017
Feb. 23, 2018
Jun. 30, 2017
Document And Entity Information [Abstract]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2017    
Entity Well-known Seasoned Issuer Yes    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Document Fiscal Year Focus 2017    
Document Fiscal Period Focus FY    
Trading Symbol AMBC    
Entity Registrant Name AMBAC FINANCIAL GROUP INC    
Entity Central Index Key 0000874501    
Current Fiscal Year End Date --12-31    
Entity Filer Category Large Accelerated Filer    
Entity Common Stock, Shares Outstanding   45,121,788  
Entity Public Float     $ 785,107,730