Income Taxes Income Taxes - NOL Usage Table (Details) - USD ($) $ in Thousands	9 Months Ended		24 Months Ended
	Sep. 30, 2017	Sep. 30, 2011	Dec. 31, 2016	Dec. 31, 2015
Net Income Loss Reconciliation [Line Items]
TierABCCredit	$ 5,000
TierABCredit	10,000
NOL allocated amount	4,259,092	$ 3,650,000
Tolling Payment to AFG			$ 100,145	$ 71,454
Ambac Assurance [Member]
Net Income Loss Reconciliation [Line Items]
Percentage Of Notional Federal Tax Liability		25.00%
AACNetTaxableIncome			1,086,124
NOL allocated amount	2,817,719
AACNetTaxableIncomeFromTierB	(253,843)		$ 607,124
AFG [Member]
Net Income Loss Reconciliation [Line Items]
Percentage Of Notional Federal Tax Liability		50.00%
NOL allocated amount	1,378,426
Tier A [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount		$ 479,000
NOL applicable percentage		15.00%
Tier B [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount		$ 1,057,000
NOL applicable percentage		40.00%
Tier C [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount		$ 1,057,000
NOL applicable percentage		10.00%
Tier D [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount		$ 1,057,000
NOL applicable percentage		15.00%
Domestic Tax Authority [Member]
Net Income Loss Reconciliation [Line Items]
NOL allocated amount	$ 4,196,145
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, 2038