Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments Held by Successor Ambac, by Contractual Maturity (Detail) (Successor Ambac [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Due in one year or less	$ 549,519
Amortized Cost, Due after one year through five years	76,880
Amortized Cost, Due after five years through ten years	116,964
Amortized Cost, Due after ten years	3,053,294
Amortized Cost, Total	3,796,657
Amortized Cost	6,485,273
Estimated Fair Value, Due in one year or less	549,500
Estimated Fair Value, Due after one year through five years	74,832
Estimated Fair Value, Due after five years through ten years	109,788
Estimated Fair Value, Due after ten years	3,007,674
Estimated Fair Value due, Total	3,741,794
Estimated Fair Value	6,448,167
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,523,755
Estimated Fair Value	1,555,612
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	91,023
Estimated Fair Value	91,055
Other Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,073,838
Estimated Fair Value	$ 1,059,706