Background and Business Description Rehabilitation Exit Support Agreement (Details)	3 Months Ended		4 Months Ended	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017 Segment	Apr. 30, 2013 $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2014	Feb. 12, 2018 USD ($)	Dec. 31, 2016 $ / shares shares	May 01, 2013 $ / shares shares
Entity Incorporation, Date of Incorporation				Apr. 29, 1991
Gain Contingency, Unrecorded Amount				$ 1,400,000,000
AAC Note Secured by RMBS Securities Value Not Less Than				$ 350,000,000
Amount of Senior Surplus Notes Outstanding on June 8, 2020				25.00%
Capital Stock Authorized | shares			150,000,000
Common stock, shares authorized | shares			130,000,000
Common Stock, Par or Stated Value Per Share | $ / shares			$ 0.01
Preferred stock, shares authorized | shares			20,000,000
Preferred Stock, Par or Stated Value Per Share | $ / shares			$ 0.01
Common stock, shares issued | shares			45,000,000
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares			$ 16.67
Investment Warrants Expiration Date			Apr. 30, 2023
Common Stock Voting Restrictions					10.00%
Common Stock Voting Restriction Less One Vote					10.00%
Common Stock Transfer Restrictions					5.00%
Common Stock Additional Transfer Restrictions					5.00%
Successor [Member]
Common stock, shares authorized | shares				130,000,000			130,000,000
Common Stock, Par or Stated Value Per Share | $ / shares				$ 0.01			$ 0.01	$ 0.01
Preferred stock, shares authorized | shares				20,000,000			20,000,000
Preferred Stock, Par or Stated Value Per Share | $ / shares				$ 0.01			$ 0.01
Common stock, shares issued | shares								45,000,000
Class of Warrant or Right, Number of Securities Called by Warrants or Rights | shares			5,047,138					5,047,138
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares								$ 16.67
Number of reportable segments | Segment		2
Ambac Assurance [Member]
Net par exposure for policies allocated to the Segregated Account				$ 9,246,357,000
AFG [Member]
Amount of Remaining Senior Suplus Notes				$ 60,000,000
Subsequent Event [Member]
Supporting Holders Participation Rate	99.60%
One Time Interest Payment on Surplus Notes						$ 13,501,000
Subsequent Event [Member] | Deferred Payment Obligation [Member]
One Dollar Outstanding						1.00
Discount Amount						0.065
Cash Payment Amount						0.40
Payment Amount in New Secured Notes						0.41
Payment Amount in Existing Surplus Notes						0.125
Subsequent Event [Member] | Surplus Notes [Member]
One Dollar Outstanding						1.00
Discount Amount						0.065
Cash Payment Amount						0.40
Payment Amount in New Secured Notes						0.41
Payment Amount in Existing Surplus Notes						0.125
Subsequent Event [Member] | Ambac [Member] | Deferred Payment Obligation [Member]
One Dollar Outstanding						1.00
Discount Amount						0.09
Payment Amount in New Secured Notes						$ 0.91