Schedule II - Income Taxes Schedule II Income Taxes (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2011	Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards		$ 4,298,770
Income Tax Examination, Penalties and Interest Accrued	$ 3,650,000
Percentage Of Notional Federal Tax Liability	25.00%
Tolling Payment Payable to AFG		70,911
TierABCCredit		5,000
TierACreditUsage		10,000
UNITED STATES
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards		129,048
United Kingdom [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards		14,936
Domestic Tax Authority [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards		4,154,786
Ambac Assurance [Member]
Operating Loss Carryforwards [Line Items]
AACNetTaxableIncome		877,313
AACNetTaxableIncomeFromTierA		479,000
AACNetTaxableIncomeFromTierB		398,313
AFG [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards		1,382,109
Tier B [Member]
Operating Loss Carryforwards [Line Items]
Operating Loss Carryforwards		$ 1,057,000