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, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Due in one year or less	$ 775,041
Amortized Cost, Due after one year through five years	1,183,781
Amortized Cost, Due after five years through ten years	1,247,527
Amortized Cost, Due after ten years	376,544
Amortized Cost, Total	3,582,893
Amortized Cost	6,556,409	6,324,568
Estimated Fair Value, Due in one year or less	775,139
Estimated Fair Value, Due after one year through five years	1,183,703
Estimated Fair Value, Due after five years through ten years	1,251,073
Estimated Fair Value, Due after ten years	383,170
Estimated Fair Value due, Total	3,593,085
Estimated Fair Value	6,771,159	6,282,658
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,664,096
Estimated Fair Value	1,824,492
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	45,876
Estimated Fair Value	46,040
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,263,544
Estimated Fair Value	$ 1,307,542