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	Jun. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Due in one year or less	$ 572,605
Amortized Cost, Due after one year through five years	1,181,838
Amortized Cost, Due after five years through ten years	1,322,550
Amortized Cost, Due after ten years	439,621
Amortized Cost, Total	3,516,614
Amortized Cost	6,277,672	6,324,568
Estimated Fair Value, Due in one year or less	572,407
Estimated Fair Value, Due after one year through five years	1,185,501
Estimated Fair Value, Due after five years through ten years	1,327,073
Estimated Fair Value, Due after ten years	449,051
Estimated Fair Value due, Total	3,534,032
Estimated Fair Value	6,509,647	6,282,658
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,686,675
Estimated Fair Value	1,853,409
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	144,386
Estimated Fair Value	144,376
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	929,997
Estimated Fair Value	$ 977,830