Background and Business Description Rehabilitation Exit Support Agreement (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2014	Dec. 31, 2018	Jun. 30, 2018	Feb. 12, 2018	Dec. 31, 2017	May 01, 2013	Apr. 30, 2013
Preferred Stock, Shares Outstanding	0		0
Entity Incorporation, Date of Incorporation	Apr. 29, 1991
satisfaction and discharge of segregated account deferred amounts					$ 3,857,000,000
Cancellation of Surplus Notes Principal					552,000,000
Cancellation of Surplus Notes Accrued Interest					257,000,000
Amount of Senior Surplus Notes Outstanding on June 8, 2020						25.00%
Common stock, shares authorized	130,000,000		130,000,000
Common Stock, Par or Stated Value Per Share	$ 0.01		$ 0.01					$ 0.01
Preferred stock, shares authorized	20,000,000		20,000,000
Preferred Stock, Par or Stated Value Per Share	$ 0.01		$ 0.01
Common stock, shares issued	45,571,743		45,365,170
Class of Warrant or Right, Exercise Price of Warrants or Rights							$ 16.67
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%
Minimum surplus amount	$ 100,000,000
Stockholders' Equity Attributable to Noncontrolling Interest	60,000,000		$ 41,000,000	$ 264,000,000
Market Value of Secured Note Collateral	$ 197,000,000		$ 210,000,000
Ambac Assurance Corporation [Member]
Preferred Stock, Shares Outstanding	5,501		4,115	26,411
Preferred Stock, Liquidation Preference, Value	$ 138,000,000		$ 103,000,000	$ 660,000,000
Ambac Assurance [Member]
Secured Notes Received from Ambac LSNI					644,000,000
Ambac [Member] | Deferred Payment Obligation [Member]
One Dollar Outstanding					1.00
Discount Amount					0.09
Payment Amount in New Secured Notes					0.91
AFG [Member]
Amount of Remaining Senior Suplus Notes						$ 60,000,000
Secured Notes Received from Ambac LSNI					125,000,000
Ambac LSNI Secured Note [Domain]
AMBAC LSNI Secured Note					$ 2,154,000,000
5.1% Surplus Notes, General Account, Due 2020 [Member] | Ambac Assurance Corporation [Member]
Debt Instrument, Interest Rate, Stated Percentage					5.10%
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
Long-term Debt [Member]
Gain Contingency, Unrecorded Amount	$ 1,400,000,000