Derivative Instruments - Summary of Notional Amounts of AFS's Trading Derivative Products (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount	$ 5,207,364	$ 11,281,777
Successor Ambac [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount	5,207,364
Successor Ambac [Member] | Interest Rate Swaps [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount	1,776,084
Successor Ambac [Member] | Interest Rate Swaps-Pay-Fixed/Receive-Variable [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount	4,635,242
Predecessor Ambac [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount		11,281,777
Predecessor Ambac [Member] | Interest Rate Swaps [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount		1,782,999
Predecessor Ambac [Member] | Interest Rate Swaps-Pay-Fixed/Receive-Variable [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount		4,707,454
AFS [Member] | Successor Ambac [Member] | Interest Rate Swaps [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount	701,218
AFS [Member] | Successor Ambac [Member] | Interest Rate Swaps-Pay-Fixed/Receive-Variable [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount	1,544,880
AFS [Member] | Successor Ambac [Member] | Interest Rate Swaps-Basis Swaps [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount	146,705
AFS [Member] | Successor Ambac [Member] | Futures Contracts [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount	100,000
AFS [Member] | Successor Ambac [Member] | Other Derivatives [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount	75,650
AFS [Member] | Predecessor Ambac [Member] | Interest Rate Swaps [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount		727,926
AFS [Member] | Predecessor Ambac [Member] | Interest Rate Swaps-Pay-Fixed/Receive-Variable [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount		1,657,382
AFS [Member] | Predecessor Ambac [Member] | Interest Rate Swaps-Basis Swaps [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount		161,690
AFS [Member] | Predecessor Ambac [Member] | Futures Contracts [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount		161,500
AFS [Member] | Predecessor Ambac [Member] | Other Derivatives [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Derivative products, Notional Amount		$ 75,651