Fresh Start Financial Statement Reporting - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended		6 Months Ended				4 Months Ended	12 Months Ended
	Jun. 30, 2013	Apr. 30, 2013	Jun. 30, 2013 Reorganization Items [Member]	Jun. 30, 2013 5.1% Notes Due 2045 [Member]	Jun. 30, 2013 Maximum [Member]	Jun. 30, 2013 Minimum [Member]	Apr. 30, 2013 Ambac Financial Group, Inc Parent Company Only [Member]	Dec. 31, 2012 Ambac Financial Group, Inc Parent Company Only [Member]
Business Acquisition, Pro Forma Information, Nonrecurring Adjustment [Line Items]
Percentage of voting shares received from emerging entity					50.00%
Enterprise value	$ 185,000	$ 185,000
Reorganization value, methodology and assumptions	The enterprise value was based on a discounted cash flow analysis using estimates of after-tax free cash flows through 2045. The terminal date of 2045 was used as the expected run-off of Ambac Assurance’s insurance operations will be substantially completed by 2045. The valuation uses a range of discount rates between 12% and 17%, which considered the cost of capital associated with a set of comparable insurance holding companies, indicative ranges suggested by third parties during Ambac’s 2008 capital raise process and discussions with financial professionals knowledgeable about the insurance industry.
Discount rate					17.00%	12.00%
Reimbursement of operating expenses			5,000
Upfront cash	30,000
Other operating payments	15,000
Junior surplus notes				350,000
Interest on junior surplus notes				5.10%
Following May 2017, additional operating expenses	4,000
Capital investment					190,000	135,000
Rate of return on investment					8.00%	6.00%
Discount rate of investment					35.00%	25.00%
Expiry year of securities	2030
Life of securities	20 years
Discontinued Liabilities subject to compromise							239,468
Liabilities subject to compromise contractual interest on debt							$ 27,572	$ 85,371