Income Taxes Income Taxes - NOL Usage Table (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	87 Months Ended
	Mar. 31, 2020	Sep. 30, 2020	Dec. 31, 2018	Dec. 31, 2011
Net Income Loss Reconciliation [Line Items]
AACNetTaxableIncome			$ 1,508
Remaining Net Operating Loss Carryforward, Amount			$ 2,142
Tolling Payment Payable to AFG, Gross		$ 28
NOL allocated amount				$ 3,650
Tier C Tolling Payments Payable to IRS		12.50%
Tier D Tolling Payments Payable to IRS		17.50%
Ambac Assurance [Member]
Net Income Loss Reconciliation [Line Items]
AACNetTaxableIncome		$ 197
NOL allocated amount		2,482
Operating Loss Generated in Current Year	$ 143
Afg [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount		1,249
Operating Loss Generated in Current Year		196
Tier A [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount		$ 479
NOL applicable percentage		15.00%
Tier B [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount		$ 1,057
NOL applicable percentage		40.00%
Tier C [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount		$ 1,057
NOL applicable percentage		10.00%
Tier D [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount		$ 1,057
NOL applicable percentage		15.00%
Domestic Tax Authority [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount		$ 3,731
Minimum [Member]
Net Income Loss Reconciliation [Line Items]
Operating Loss Carryforwards, Expiration Date		Dec. 31, 2029
Maximum [Member]
Net Income Loss Reconciliation [Line Items]
Operating Loss Carryforwards, Expiration Date		Dec. 31, 2040