Income Taxes Income Taxes - NOL Usage Table (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended			36 Months Ended	48 Months Ended
	Sep. 30, 2018	Sep. 30, 2011	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2016
Net Income Loss Reconciliation [Line Items]
TierABCCredit	$ 5,000				$ 5,000
TierABCredit				$ (10,000)
NOL allocated amount		$ 3,650,000	$ 3,423,704				$ 3,423,704
Tolling Payment to AFG					$ 71,454			$ 100,145
Tolling Payment Payable to AFG			44,381	$ 30,496		$ 30,496	44,381
Tolling Payment Payable to AFG			$ 13,885				$ 13,885	$ 28,691
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						1,486,879
NOL allocated amount			$ 2,163,121				$ 2,163,121
AACNetTaxableIncomeFromTierB						$ 1,007,879	1,007,879
Percentage Of Notional Federal Tax Liability		25.00%
AFG [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount			1,260,583				1,260,583
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%