Fair Value Measurements - Summary of Gains and Losses (Realized and Unrealized) Relating to Level 3 Assets and Liabilities Included in Earnings (Detail) (USD $) In Thousands, unless otherwise specified	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	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	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
	Sep. 30, 2013 Successor Ambac [Member] Variable Interest Entity [Member]	Sep. 30, 2013 Successor Ambac [Member] Variable Interest Entity [Member]	Sep. 30, 2012 Predecessor Ambac [Member] Variable Interest Entity [Member]	Apr. 30, 2013 Predecessor Ambac [Member] Variable Interest Entity [Member]	Sep. 30, 2012 Predecessor Ambac [Member] Variable Interest Entity [Member]	Sep. 30, 2013 Realized Gains Or (Losses) and Other Settlements On Credit Derivative Contracts [Member] Successor Ambac [Member]	Sep. 30, 2013 Realized Gains Or (Losses) and Other Settlements On Credit Derivative Contracts [Member] Successor Ambac [Member]	Sep. 30, 2012 Realized Gains Or (Losses) and Other Settlements On Credit Derivative Contracts [Member] Predecessor Ambac [Member]	Apr. 30, 2013 Realized Gains Or (Losses) and Other Settlements On Credit Derivative Contracts [Member] Predecessor Ambac [Member]	Sep. 30, 2012 Realized Gains Or (Losses) and Other Settlements On Credit Derivative Contracts [Member] Predecessor Ambac [Member]	Sep. 30, 2013 Unrealized Gains Or (Losses) On Credit Derivative Contracts [Member] Successor Ambac [Member]	Sep. 30, 2013 Unrealized Gains Or (Losses) On Credit Derivative Contracts [Member] Successor Ambac [Member]	Sep. 30, 2012 Unrealized Gains Or (Losses) On Credit Derivative Contracts [Member] Predecessor Ambac [Member]	Apr. 30, 2013 Unrealized Gains Or (Losses) On Credit Derivative Contracts [Member] Predecessor Ambac [Member]	Sep. 30, 2012 Unrealized Gains Or (Losses) On Credit Derivative Contracts [Member] Predecessor Ambac [Member]	Sep. 30, 2013 Interest Rate Swaps [Member] Successor Ambac [Member]	Sep. 30, 2013 Interest Rate Swaps [Member] Successor Ambac [Member]	Sep. 30, 2012 Interest Rate Swaps [Member] Predecessor Ambac [Member]	Apr. 30, 2013 Interest Rate Swaps [Member] Predecessor Ambac [Member]	Sep. 30, 2012 Interest Rate Swaps [Member] Predecessor Ambac [Member]	Sep. 30, 2013 Other Income or (Loss) [Member] Successor Ambac [Member]	Sep. 30, 2013 Other Income or (Loss) [Member] Successor Ambac [Member]	Sep. 30, 2012 Other Income or (Loss) [Member] Predecessor Ambac [Member]	Apr. 30, 2013 Other Income or (Loss) [Member] Predecessor Ambac [Member]	Sep. 30, 2012 Other Income or (Loss) [Member] Predecessor Ambac [Member]	Sep. 30, 2013 Net Investment Income [Member] Successor Ambac [Member]	Sep. 30, 2013 Net Investment Income [Member] Successor Ambac [Member]	Sep. 30, 2012 Net Investment Income [Member] Predecessor Ambac [Member]	Apr. 30, 2013 Net Investment Income [Member] Predecessor Ambac [Member]	Sep. 30, 2012 Net Investment Income [Member] Predecessor Ambac [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total gains or losses included in earnings for the period																										$ 467	$ 863	$ (22)	$ (33)	$ 17
Total gains or losses included in earnings for the period	253,815	(537,477)	341,578	1,146,256	462,009	1,581	7,655	2,945	3,444	9,271	29,614	74,760	24,496	(63,828)	3,532	5,931	24,838	(1,156)	(28,162)	(46,191)	(213)	(390)	(771)	(496)	98,952
Changes in unrealized gains or losses relating to the assets and liabilities still held at the reporting date	$ 253,815	$ (537,477)	$ 341,578	$ 1,146,256	$ 462,490						$ 27,053	$ 40,671	$ 23,948	$ (63,840)	$ (4,192)	$ 5,931	$ 24,838	$ (1,156)	$ (28,162)	$ (47,419)	$ (213)	$ (390)	$ (771)	$ (496)	$ (1,758)