Background and Business Description Rehabilitation Exit Support Agreement (Details) - USD ($)	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Feb. 12, 2018
Cancellation of Surplus Notes and Accrued Interest				$ 552,320,000
Cancellation of Surplus Notes Accrued Interest				257,200,000
SatisfactionAndDischargeOfSegregatedAccountDeferredAmounts				3,856,992,000
Gain Contingency, Unrecorded Amount	$ 1,400,000,000	$ 1,400,000,000
Market Value of Secured Note Collateral	255,628,000	255,628,000
Tier 2 Notes				240,000,000
Tier 2 Note Security in RMBS Litigation Settlement Proceeds				1,600,000,000
One Time Interest Payment on Surplus Notes				13,501,000
Successor [Member]
Long-term debt	2,796,389,000	2,796,389,000	$ 991,696,000
Deferred Payment Obligation [Member]
Discount Amount				0.065
Surplus Notes [Member]
Cash Payment Amount				0.40
Payment Amount in New Secured Notes				0.41
Payment Amount in Existing Surplus Notes				0.125
Ambac LSNI Secured Note [Domain]
AMBAC LSNI Secured Note				2,154,332,000
Ambac Assurance Corporation [Member] | Successor [Member]
Long-term debt	$ 2,796,389,000	$ 2,796,389,000	$ 991,696,000
Ambac Assurance [Member]
Secured Notes Received from Ambac LSNI				643,583,000
Ambac [Member] | Deferred Payment Obligation [Member]
One Dollar Outstanding				1.00
Payment Amount in New Secured Notes				0.91
Discount Amount				0.09
Afg [Member]
Secured Notes Received from Ambac LSNI				124,881,000
Ambac [Member]
One Time Interest Payment on Surplus Notes				$ 2,618,000
5.1% Surplus Notes, General Account, Due 2020 [Member] | Ambac Assurance Corporation [Member] | Successor [Member]
Debt Instrument, Interest Rate, Stated Percentage	5.10%	5.10%	5.10%
Debt Instruments Maturity Year	2020	2020	2020
Ambac Note Post Exit From Rehabilitation [Member] | Ambac Assurance Corporation [Member] | Successor [Member]
Long-term debt	$ 2,038,912,000	$ 2,038,912,000	$ 0