Special Purpose Entities, Including Variable Interest Entities - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	5 Months Ended	9 Months Ended			3 Months Ended	5 Months Ended	3 Months Ended	4 Months Ended	9 Months Ended		3 Months Ended	5 Months Ended	3 Months Ended	4 Months Ended	9 Months Ended					9 Months Ended	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Apr. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Successor Ambac [Member]	Sep. 30, 2013 Successor Ambac [Member]	Sep. 30, 2012 Predecessor Ambac [Member]	Apr. 30, 2013 Predecessor Ambac [Member]	Sep. 30, 2012 Predecessor Ambac [Member]	Sep. 30, 2013 Asset-Backed Securities and Utility Obligations [Member]	Sep. 30, 2013 Financial Guarantee [Member] Successor Ambac [Member]	Sep. 30, 2013 Financial Guarantee [Member] Successor Ambac [Member]	Sep. 30, 2012 Financial Guarantee [Member] Predecessor Ambac [Member]	Apr. 30, 2013 Financial Guarantee [Member] Predecessor Ambac [Member]	Sep. 30, 2012 Financial Guarantee [Member] Predecessor Ambac [Member]	Sep. 30, 2013 Sponsored Variable Interest Entities [Member] Transactions	Dec. 31, 2012 Sponsored Variable Interest Entities [Member]	Sep. 30, 2013 Variable Interest Entities [Member]	Dec. 31, 2012 Variable Interest Entities [Member]	Sep. 30, 2013 Consolidated Entities [Member] Entity	Dec. 31, 2012 Consolidated Entities [Member] Entity
Variable Interest Entities [Line Items]
Debt outstanding to third parties under secured borrowing transaction																			$ 1,503	$ 14,588
Securities fair value	230,049	230,049	230,049		201,329														230,049	201,329
Fair value of special purpose entities	13,671	13,671	13,671		14,557
Number of individual transactions with special purpose entities																	15
Number of transactions outstanding with special purpose entities																	3
Total principal amount of debt outstanding																	464,495	466,938
Weighted average life											8 years 8 months 12 days
Average rating of assets held by sponsored special purpose entities			BBB+
Earned premiums for issuing the financial guarantee policies on the assets, MTNs and derivative contracts						30	50	87	40	342
Assets sold to special purpose entities						2,747	2,747	4,846	1,455	14,691
Claims on insurance policies issued to these entities			0
Earned fees for providing other services under its financial guarantee policies												2	4	10	3	32
Amount received under derivative contracts							1,134			3,387
Amount paid under derivative contracts						88		360	93
Number of consolidated Variable Interest Entities																					19	18
Consolidated VIE assets	17,386,418	17,386,418	17,386,418		17,841,863
Consolidated VIE liabilities	17,324,261	17,324,261	17,324,261		17,661,700
Intangible assets recorded at fair value				164,520
Weighted-average amortization period			16 years
Amortization expense for intangible assets	$ 2,507	$ 5,015