Long-Term Debt - Carrying Value of Long-Term Debt (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Long-term debt	$ 150,170	$ 223,601
Variable interest entity [Member]
Debt Instrument [Line Items]
Long-term debt	15,436,008	14,288,540
5.1% surplus notes, general account, due 2020 [Member] | Ambac Assurance Corporation [Member]
Debt Instrument [Line Items]
Long-term debt	139,476	213,790
5.1% surplus notes, segregated account, due 2020 [Member] | Ambac Assurance Corporation [Member]
Debt Instrument [Line Items]
Long-term debt	6,502	6,020
5.1% junior surplus notes, segregated account, due 2020 [Member] | Ambac Assurance Corporation [Member]
Debt Instrument [Line Items]
Long-term debt	$ 4,192	$ 3,791