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	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Due in one year or less	$ 461,954
Amortized Cost, Due after one year through five years	1,237,724
Amortized Cost, Due after five years through ten years	287,768
Amortized Cost, Due after ten years	833,297
Amortized Cost, Total	2,820,743
Amortized Cost	4,119,358	$ 5,271,818
Estimated Fair Value, Due in one year or less	461,683
Estimated Fair Value, Due after one year through five years	1,239,764
Estimated Fair Value, Due after five years through ten years	279,803
Estimated Fair Value, Due after ten years	832,145
Estimated Fair Value due, Total	2,813,395
Fixed income securities	4,277,297	5,309,161
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,713,908	4,614,623
Fixed income securities	3,871,938	4,652,172
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	726,998	2,214,512
Fixed income securities	823,296	2,251,333
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	50,754	50,754
Fixed income securities	50,958	51,037
Fixed Income Securities [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	520,863	531,660
Fixed income securities	$ 589,648	$ 597,942