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 [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Due in one year or less	$ 551,712
Amortized Cost, Due after one year through five years	845,986
Amortized Cost, Due after five years through ten years	931,471
Amortized Cost, Due after ten years	198,385
Amortized Cost, Total	2,527,554
Amortized Cost	4,939,325	6,324,568
Estimated Fair Value, Due in one year or less	551,618
Estimated Fair Value, Due after one year through five years	846,430
Estimated Fair Value, Due after five years through ten years	935,797
Estimated Fair Value, Due after ten years	202,095
Estimated Fair Value due, Total	2,535,940
Fixed income securities	5,150,018	6,282,658
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,557,059
Fixed income securities	1,710,955
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,346
Fixed income securities	21,122
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	833,366
Fixed income securities	$ 882,001