Schedule II - Income Taxes Schedule II Income Taxes (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended			24 Months Ended	36 Months Ended
	Sep. 30, 2011	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards	$ 3,650,000	$ 3,694,844				$ 3,694,844
Tolling Payment Payable to AFG		30,496	$ 28,691		$ 28,691	30,496
TierABCCredit		5,000		$ 5,000
Domestic Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards		3,694,844				3,694,844
Ambac Assurance [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards		2,282,205				2,282,205
Percentage Of Notional Federal Tax Liability	25.00%
AACNetTaxableIncome						1,367,795
AACNetTaxableIncomeFromTierA			$ 479,000
AACNetTaxableIncomeFromTierB					$ 607,124	888,795
AFG [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards		1,412,639				1,412,639
Percentage Of Notional Federal Tax Liability	50.00%
Tier B [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards	$ 1,057,000	$ 1,057,000				$ 1,057,000
Maximum [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards, Expiration Date		Dec. 31, 2032