Long-term Debt - Carrying Value of Long-Term Debt (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Long-term debt	$ 146,909	$ 223,601
Variable Interest Entity [Member]
Debt Instrument [Line Items]
Long-term debt	15,113,094	14,288,540
5.1% Surplus Notes, General Account, Due 2020 [Member] | Ambac Assurance Corporation [Member]
Debt Instrument [Line Items]
Long-term debt	136,469	213,790
5.1% Surplus Notes, Segregated Account, Due 2020 [Member] | Ambac Assurance Corporation [Member]
Debt Instrument [Line Items]
Long-term debt	6,359	6,020
5.1% Junior Surplus Notes, Segregated Account, Due 2020 [Member] | Ambac Assurance Corporation [Member]
Debt Instrument [Line Items]
Long-term debt	$ 4,081	$ 3,791