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	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Due in one year or less	$ 928,100
Amortized Cost, Due after one year through five years	771,616
Amortized Cost, Due after five years through ten years	544,655
Amortized Cost, Due after ten years	508,605
Amortized Cost, Total	2,752,976
Amortized Cost	5,641,645	$ 5,931,045
Estimated Fair Value, Due in one year or less	928,060
Estimated Fair Value, Due after one year through five years	775,497
Estimated Fair Value, Due after five years through ten years	546,607
Estimated Fair Value, Due after ten years	510,532
Estimated Fair Value due, Total	2,760,696
Fixed income securities	5,794,058	6,049,908
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,825,555	5,435,385
Fixed income securities	4,978,118	5,554,215
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,197,085	2,284,425
Fixed income securities	2,279,639	2,351,595
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	90,556	113,650
Fixed income securities	90,925	113,923
Fixed Income Securities [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	601,028	778,383
Fixed income securities	$ 662,798	$ 828,783