Income Taxes Income Taxes - NOL Usage Table (Details) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2011
Net Income Loss Reconciliation [Line Items]
Impact of AMT Repeal on Current Taxes		$ 29,581
Change in Impact of AMT Repeal on Current Taxes	$ 1,902
AMT Related to Filing of 2017 Tax Return		5,536
AMT Receivable Resulting from TCJA				$ 37,019
Net Impact of New Tax Law		$ 1,886
NOL allocated amount			$ 3,639,000		$ 3,440,000
Amount of NOL Reallocated from AAC to AFG			210,000
Tolling Payments Paid by AAC to AFG			28,000
AFG [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount			$ 1,457,000
Minimum [Member]
Net Income Loss Reconciliation [Line Items]
Operating Loss Carryforwards, Expiration Date			Dec. 31, 2029
Minimum [Member] | Ambac Assurance [Member]
Net Income Loss Reconciliation [Line Items]
Operating Loss Carryforwards, Expiration Date			Dec. 31, 2029
Minimum [Member] | AFG [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
Potential Tolling Payment From AAC to IRS			$ 8,000
Maximum [Member] | Ambac Assurance [Member]
Net Income Loss Reconciliation [Line Items]
Operating Loss Carryforwards, Expiration Date			Dec. 31, 2040
Maximum [Member] | AFG [Member]
Net Income Loss Reconciliation [Line Items]
Operating Loss Carryforwards, Expiration Date			Jan. 01, 2034