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, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Due in one year or less	$ 714,761
Amortized Cost, Due after one year through five years	556,370
Amortized Cost, Due after five years through ten years	376,420
Amortized Cost, Due after ten years	827,341
Amortized Cost, Total	2,474,892
Amortized Cost	5,271,818	$ 5,931,045
Estimated Fair Value, Due in one year or less	714,507
Estimated Fair Value, Due after one year through five years	556,387
Estimated Fair Value, Due after five years through ten years	376,686
Estimated Fair Value, Due after ten years	761,269
Estimated Fair Value due, Total	2,408,849
Fixed income securities	$ 5,309,161	$ 6,049,908