Income Taxes Income Taxes - NOL Usage Table (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended		24 Months Ended	36 Months Ended
	Sep. 30, 2011	Dec. 31, 2017	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2017
Net Income Loss Reconciliation [Line Items]
TierABCCredit		$ 5,000	$ 5,000
TierABCredit		(10,000)
NOL allocated amount	$ 3,650,000	3,694,844			$ 3,694,844
Tolling Payment to AFG		637	$ 71,454	$ 100,145	637
Tolling Payment Payable to AFG		30,496		28,691	30,496
Tolling Payment Payable to AFG		$ 31,133		28,691	$ 31,133
Tier C Tolling Payment Payable to IRS		12.50%			12.50%
Tier D Tolling Payments Payable to IRS		17.50%			17.50%
Ambac Assurance [Member]
Net Income Loss Reconciliation [Line Items]
AACNetTaxableIncome					$ 1,367,795
NOL allocated amount		$ 2,282,205			2,282,205
AACNetTaxableIncomeFromTierB				$ 607,124	888,795
Percentage Of Notional Federal Tax Liability	25.00%
AFG [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount		1,412,639			1,412,639
Percentage Of Notional Federal Tax Liability	50.00%
Tier A [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount	$ 479,000	$ 479,000			479,000
NOL applicable percentage		15.00%
Tier B [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount	$ 1,057,000	$ 1,057,000			1,057,000
NOL applicable percentage		40.00%
Tier C [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount		$ 1,057,000			1,057,000
NOL applicable percentage		10.00%
Tier D [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount		$ 1,057,000			$ 1,057,000
NOL applicable percentage		15.00%