Income Taxes Income Taxes - NOL Usage Table (Details) - USD ($) $ in Millions	3 Months Ended	87 Months Ended
	Mar. 31, 2020	Dec. 31, 2018	Sep. 30, 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	$ 172
NOL allocated amount	2,457
Operating Loss Generated in Current Year	143
Afg [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount	1,246
Operating Loss Generated in Current Year	168
Domestic Tax Authority [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount	$ 3,703
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