Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013 Successor Ambac [Member]	Sep. 30, 2013 Successor Ambac [Member] Corporate Obligations [Member]	Sep. 30, 2013 Successor Ambac [Member] Residential Mortgage-Backed Securities [Member]	Sep. 30, 2013 Successor Ambac [Member] Collateralized Debt Obligations [Member]	Sep. 30, 2013 Successor Ambac [Member] Short-Term [Member]	Sep. 30, 2013 Successor Ambac [Member] Fixed Income Securities [Member]	Sep. 30, 2013 Successor Ambac [Member] Fixed Income Securities [Member] Securities Pledged as Collateral [Member]	Sep. 30, 2013 Successor Ambac [Member] Fixed Income Securities [Member] Municipal Obligations [Member]	Sep. 30, 2013 Successor Ambac [Member] Fixed Income Securities [Member] Corporate Obligations [Member]	Sep. 30, 2013 Successor Ambac [Member] Fixed Income Securities [Member] Foreign Obligations [Member]	Sep. 30, 2013 Successor Ambac [Member] Fixed Income Securities [Member] U.S. Government Obligations [Member]	Sep. 30, 2013 Successor Ambac [Member] Fixed Income Securities [Member] U.S. Government Obligations [Member] Securities Pledged as Collateral [Member]	Sep. 30, 2013 Successor Ambac [Member] Fixed Income Securities [Member] U.S. Agency Obligations [Member]	Sep. 30, 2013 Successor Ambac [Member] Fixed Income Securities [Member] Residential Mortgage-Backed Securities [Member]	Sep. 30, 2013 Successor Ambac [Member] Fixed Income Securities [Member] Collateralized Debt Obligations [Member]	Sep. 30, 2013 Successor Ambac [Member] Fixed Income Securities [Member] Asset-Backed Securities [Member]	Sep. 30, 2013 Successor Ambac [Member] Fixed Income Investments And Other Investments [Member]	Dec. 31, 2012 Predecessor Ambac [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Corporate Obligations [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Short-Term [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Fixed Income Securities [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Fixed Income Securities [Member] Securities Pledged as Collateral [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Fixed Income Securities [Member] Municipal Obligations [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Fixed Income Securities [Member] Corporate Obligations [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Fixed Income Securities [Member] Foreign Obligations [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Fixed Income Securities [Member] U.S. Government Obligations [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Fixed Income Securities [Member] U.S. Government Obligations [Member] Securities Pledged as Collateral [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Fixed Income Securities [Member] U.S. Agency Obligations [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Fixed Income Securities [Member] Residential Mortgage-Backed Securities [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Fixed Income Securities [Member] Collateralized Debt Obligations [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Fixed Income Securities [Member] Asset-Backed Securities [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Fixed Income Investments And Other Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 6,485,273		$ 1,523,755	$ 91,023	$ 549,233	$ 5,795,824	$ 140,216	$ 1,470,363	$ 1,367,029	$ 127,649	$ 106,605	$ 140,216	$ 35,562	$ 1,523,755	$ 91,023	$ 1,073,838	$ 6,345,057	$ 5,678,560		$ 661,219	$ 4,751,824	$ 265,517	$ 1,662,124	$ 999,554	$ 67,347	$ 127,037	$ 265,517	$ 79,295	$ 1,096,202	$ 32,855	$ 687,410	$ 5,413,043
Gross Unrealized Gains	64,647				1	64,589	57	152	1,981	56	36	57	4	52,896	256	9,208	64,590	728,987		439	728,286	262	187,191	87,535	2,765	872	262	3,240	379,935	1,015	65,733	728,725
Gross Unrealized Losses	101,753				5	101,748		26,157	24,324	5,206	1,381		77	21,039	224	23,340	101,753	77,715			77,715		383	9,117		626			20,555	528	46,506	77,715
Estimated Fair Value	6,448,167	2,423,914	1,555,612	91,055	549,229	5,758,665	140,273	1,444,358	1,344,686	122,499	105,260	140,273	35,489	1,555,612	91,055	1,059,706	6,307,894	6,329,832	2,261,294	661,658	5,402,395	265,779	1,848,932	1,077,972	70,112	127,283	265,779	82,535	1,455,582	33,342	706,637	6,064,053
Non - credit other - than - temporary Impairments	$ 947					$ 947								$ 947			$ 947	$ 6,892			$ 6,892								$ 6,892			$ 6,892