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, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Due in one year or less	$ 553,892
Amortized Cost, Due after one year through five years	1,216,940
Amortized Cost, Due after five years through ten years	837,225
Amortized Cost, Due after ten years	146,530
Amortized Cost, Total	2,754,587
Amortized Cost	5,931,045	$ 5,283,157
Estimated Fair Value, Due in one year or less	554,454
Estimated Fair Value, Due after one year through five years	1,220,450
Estimated Fair Value, Due after five years through ten years	834,775
Estimated Fair Value, Due after ten years	145,928
Estimated Fair Value due, Total	2,755,607
Fixed income securities	6,049,908	$ 5,334,120
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,284,425
Fixed income securities	2,351,595
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	113,650
Fixed income securities	113,923
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	778,383
Fixed income securities	$ 828,783