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	Mar. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Due in one year or less	$ 512,768
Amortized Cost, Due after one year through five years	1,127,009
Amortized Cost, Due after five years through ten years	1,326,425
Amortized Cost, Due after ten years	502,249
Amortized Cost, Total	3,468,451
Amortized Cost	6,462,103	6,324,568
Estimated Fair Value, Due in one year or less	512,927
Estimated Fair Value, Due after one year through five years	1,124,957
Estimated Fair Value, Due after five years through ten years	1,310,305
Estimated Fair Value, Due after ten years	505,736
Estimated Fair Value due, Total	3,453,925
Estimated Fair Value	6,508,915	6,282,658
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,578,205
Estimated Fair Value	1,655,658
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	177,780
Estimated Fair Value	177,339
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,237,667
Estimated Fair Value	$ 1,221,993