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	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Due in one year or less	$ 435,165
Amortized Cost, Due after one year through five years	1,121,578
Amortized Cost, Due after five years through ten years	848,880
Amortized Cost, Due after ten years	148,666
Amortized Cost, Total	2,554,289
Amortized Cost	5,951,830	$ 5,283,157
Estimated Fair Value, Due in one year or less	435,941
Estimated Fair Value, Due after one year through five years	1,145,770
Estimated Fair Value, Due after five years through ten years	881,439
Estimated Fair Value, Due after ten years	158,402
Estimated Fair Value due, Total	2,621,552
Fixed income securities	6,110,754	5,334,120
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,550,887	4,992,756
Fixed income securities	5,709,464	5,043,776
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,221,276	1,942,285
Fixed income securities	2,270,543	1,977,338
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	105,574	85,706
Fixed income securities	104,562	84,267
Fixed Income Securities [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,070,691	802,842
Fixed income securities	$ 1,114,097	$ 840,527