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, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Due in one year or less	$ 536,088
Amortized Cost, Due after one year through five years	1,061,175
Amortized Cost, Due after five years through ten years	864,905
Amortized Cost, Due after ten years	174,700
Amortized Cost, Total	2,636,868
Amortized Cost	6,041,185	$ 5,283,157
Estimated Fair Value, Due in one year or less	536,300
Estimated Fair Value, Due after one year through five years	1,070,684
Estimated Fair Value, Due after five years through ten years	881,715
Estimated Fair Value, Due after ten years	179,389
Estimated Fair Value due, Total	2,668,088
Fixed income securities	6,151,939	5,334,120
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,539,759	4,992,756
Fixed income securities	5,650,261	5,043,776
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,229,764	1,942,285
Fixed income securities	2,274,055	1,977,338
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	100,949	85,706
Fixed income securities	99,341	84,267
Fixed Income Securities [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,073,604	802,842
Fixed income securities	$ 1,110,455	$ 840,527