Income Taxes Income Taxes - NOL Usage Table (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2018	Sep. 30, 2011	Dec. 31, 2017
Net Income Loss Reconciliation [Line Items]
Tolling Payment Payable to AFG	$ 14,129
NOL allocated amount		$ 3,650,000
Tier C Credit	(5,000)
AAC Remaining Taxable Income From Tier C to Absorb Credit	69
Ambac Assurance [Member]
Net Income Loss Reconciliation [Line Items]
Percentage Of Notional Federal Tax Liability		25.00%
NOL allocated amount	1,875,973
AACNetTaxableIncomeFromTierB			$ (406,232)
AAC Remaining Taxable Income From Tier B	168,205
AAC Net Taxable Income From Tier C	238,026
AFG [Member]
Net Income Loss Reconciliation [Line Items]
Percentage Of Notional Federal Tax Liability		50.00%
NOL allocated amount	1,403,518
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	$ 3,279,491
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, 2032