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, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Due in one year or less	$ 528,764
Amortized Cost, Due after one year through five years	1,178,233
Amortized Cost, Due after five years through ten years	292,286
Amortized Cost, Due after ten years	804,689
Amortized Cost, Total	2,803,972
Amortized Cost	3,833,380	$ 5,271,818
Estimated Fair Value, Due in one year or less	528,756
Estimated Fair Value, Due after one year through five years	1,182,221
Estimated Fair Value, Due after five years through ten years	285,711
Estimated Fair Value, Due after ten years	839,164
Estimated Fair Value due, Total	2,835,852
Fixed income securities	3,993,015	5,309,161
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,355,223	4,614,623
Fixed income securities	3,514,927	4,652,172
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	460,252	2,214,512
Fixed income securities	512,309	2,251,333
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	65,255	50,754
Fixed income securities	65,457	51,037
Fixed Income Securities [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	503,901	531,660
Fixed income securities	$ 579,397	$ 597,942