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, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Due in one year or less	$ 365,867
Amortized Cost, Due after one year through five years	915,796
Amortized Cost, Due after five years through ten years	1,024,407
Amortized Cost, Due after ten years	192,402
Amortized Cost, Total	2,498,472
Amortized Cost	5,044,988	$ 4,939,325
Estimated Fair Value, Due in one year or less	365,798
Estimated Fair Value, Due after one year through five years	912,985
Estimated Fair Value, Due after five years through ten years	1,012,146
Estimated Fair Value, Due after ten years	187,931
Estimated Fair Value due, Total	2,478,860
Fixed income securities	5,124,394	5,150,018
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,813,186	4,514,878
Fixed income securities	4,892,421	4,725,686
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,881,009	1,557,059
Fixed income securities	1,963,408	1,710,955
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	86,834	21,346
Fixed income securities	86,627	21,122
Fixed Income Securities [Member] | Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	578,673	833,366
Fixed income securities	$ 595,499	$ 882,001