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	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Due in one year or less	$ 347,240
Amortized Cost, Due after one year through five years	1,020,940
Amortized Cost, Due after five years through ten years	914,404
Amortized Cost, Due after ten years	169,740
Amortized Cost, Total	2,452,324
Amortized Cost	5,283,157	$ 4,939,325
Estimated Fair Value, Due in one year or less	347,359
Estimated Fair Value, Due after one year through five years	1,014,416
Estimated Fair Value, Due after five years through ten years	901,252
Estimated Fair Value, Due after ten years	168,961
Estimated Fair Value due, Total	2,431,988
Fixed income securities	5,334,120	$ 5,150,018
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,942,285
Fixed income securities	1,977,338
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	85,706
Fixed income securities	84,267
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	802,842
Fixed income securities	$ 840,527