Insurance Regulatory Restrictions - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Insurance Regulatory Restrictions [Line Items]
Qualified statutory capital	$ 628,302	$ 684,610
Average rates of return on its admitted assets	8.01%	7.45%
Ambac Assurance [Member]
Insurance Regulatory Restrictions [Line Items]
Segregated account reported statutory policyholder surplus	100,000	495,293
Segregated Account [Member]
Insurance Regulatory Restrictions [Line Items]
Segregated account reported statutory policyholder surplus	(61,812)	105,899
Segregated Account [Member] | Ambac Assurance [Member]
Insurance Regulatory Restrictions [Line Items]
Segregated account's insurance liabilities	163,687
Restricted stock and restricted stock units RSU [Member]
Insurance Regulatory Restrictions [Line Items]
Policyholders' surplus percentage	10.00%
Performance units [Member]
Insurance Regulatory Restrictions [Line Items]
Discounting loss reserves and estimated impairment losses on subsidiary guarantees	5.10%
Performance units [Member] | Minimum [Member]
Insurance Regulatory Restrictions [Line Items]
Quarterly dividend percentage	15.00%
Performance units [Member] | Maximum [Member]
Insurance Regulatory Restrictions [Line Items]
Cash dividends paid on common stock	0	0
Prescribed Practice Discount Rate [Member]
Insurance Regulatory Restrictions [Line Items]
Statutory surplus	117,602	98,225
Prescribed practice surplus notes value [Member]
Insurance Regulatory Restrictions [Line Items]
Statutory surplus	$ 35,761	$ 84,344