Special Purpose Entities, Including Variable Interest Entities - Summary of Fair Value of Fixed Income Securities, by Asset-Type, Held by Consolidated Variable Interest Entities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013 Successor Ambac [Member]	Sep. 30, 2013 Successor Ambac [Member] Variable Interest Entities [Member]	Dec. 31, 2012 Predecessor Ambac [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Variable Interest Entities [Member]	Sep. 30, 2013 Corporate Obligations [Member] Successor Ambac [Member]	Dec. 31, 2012 Corporate Obligations [Member] Predecessor Ambac [Member]
Investments:
Fixed income securities	$ 6,448,167	$ 2,423,914	$ 6,329,832	$ 2,261,294	$ 2,423,914	$ 2,261,294