Schedule II - Condensed Financial Information of Registrant (Parent Company Only) Schedule II - Condensed Financial Information of Registrant - Income Taxes (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended	87 Months Ended
	Sep. 30, 2011	Dec. 31, 2019	Dec. 31, 2018
Operating Loss Carryforwards	$ 3,650	$ 3,535
AACNetTaxableIncome			$ 1,508
Operating Loss Generated in Current Year		143
AACNetTaxableIncomeFromTierB		1,029
Tolling Payment Payable to AFG		147
Tolling Payment to AFG		119
TierABCCredit		(5)
TierABCredit		(10)
Tolling Payment Payable to AFG Outstanding Amount		$ 28
Minimum [Member]
Operating Loss Carryforwards, Expiration Date		Dec. 31, 2029
Maximum [Member]
Operating Loss Carryforwards, Expiration Date		Dec. 31, 2040
AFG [Member]
Operating Loss Carryforwards		$ 1,250
AFG [Member] | Minimum [Member]
Operating Loss Carryforwards, Expiration Date		Dec. 31, 2029
AFG [Member] | Maximum [Member]
Operating Loss Carryforwards, Expiration Date		Jan. 01, 2034
Ambac Assurance [Member]
Operating Loss Carryforwards		$ 2,285
Percentage Of Notional Federal Tax Liability	25.00%
Ambac Assurance [Member] | Minimum [Member]
Operating Loss Carryforwards, Expiration Date		Dec. 31, 2029
Ambac Assurance [Member] | Maximum [Member]
Operating Loss Carryforwards, Expiration Date		Dec. 31, 2040
Tier B [Member]
Operating Loss Carryforwards		$ 1,057