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	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Due in one year or less	$ 629,872
Amortized Cost, Due after one year through five years	947,797
Amortized Cost, Due after five years through ten years	721,951
Amortized Cost, Due after ten years	490,563
Amortized Cost, Total	2,790,183
Amortized Cost	5,734,282	$ 5,931,045
Estimated Fair Value, Due in one year or less	629,937
Estimated Fair Value, Due after one year through five years	954,210
Estimated Fair Value, Due after five years through ten years	728,627
Estimated Fair Value, Due after ten years	489,458
Estimated Fair Value due, Total	2,802,232
Fixed income securities	5,887,129	6,049,908
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,165,245	5,435,385
Fixed income securities	5,318,195	5,554,215
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,220,640	2,284,425
Fixed income securities	2,299,528	2,351,595
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	124,913	113,650
Fixed income securities	125,106	113,923
Fixed Income Securities [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	598,546	778,383
Fixed income securities	$ 660,263	$ 828,783