Schedule II - Income Taxes Schedule II Income Taxes (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended		24 Months Ended
	Sep. 30, 2011	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards		$ 4,027,329		$ 4,027,329
Income Tax Examination, Penalties and Interest Accrued	$ 3,650,000
Tolling Payment Payable to AFG		28,691	$ 70,911	28,691
TierABCCredit			5,000
UNITED STATES
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards		74,094		74,094
Domestic Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards		3,953,235		3,953,235
Ambac Assurance [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards		2,563,876		2,563,876
Percentage Of Notional Federal Tax Liability	25.00%
AACNetTaxableIncome		1,086,124	877,313
AACNetTaxableIncomeFromTierA		479,000	479,000
AACNetTaxableIncomeFromTierB		204,932	$ 402,192	607,124
AFG [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards		1,389,359		1,389,359
Percentage Of Notional Federal Tax Liability	50.00%
Tier B [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards		$ 1,057,000		$ 1,057,000