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, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Due in one year or less	$ 482,916
Amortized Cost, Due after one year through five years	1,213,216
Amortized Cost, Due after five years through ten years	752,256
Amortized Cost, Due after ten years	265,308
Amortized Cost, Total	2,713,696
Amortized Cost	5,715,764	$ 5,931,045
Estimated Fair Value, Due in one year or less	483,125
Estimated Fair Value, Due after one year through five years	1,218,029
Estimated Fair Value, Due after five years through ten years	753,397
Estimated Fair Value, Due after ten years	264,533
Estimated Fair Value due, Total	2,719,084
Fixed income securities	5,855,962	6,049,908
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,303,723	5,435,385
Fixed income securities	5,443,999	5,554,215
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,240,984	2,284,425
Fixed income securities	2,314,487	2,351,595
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	152,507	113,650
Fixed income securities	153,104	113,923
Fixed Income Securities [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	608,577	778,383
Fixed income securities	$ 669,287	$ 828,783