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 Ambac [Member], USD $) In Thousands, unless otherwise specified	Jun. 30, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Due in one year or less	$ 1,037,466
Amortized Cost, Due after one year through five years	834,750
Amortized Cost, Due after five years through ten years	1,179,225
Amortized Cost, Due after ten years	842,884
Amortized Cost, Total	3,894,325
Amortized Cost	6,451,730
Estimated Fair Value, Due in one year or less	1,037,376
Estimated Fair Value, Due after one year through five years	823,355
Estimated Fair Value, Due after five years through ten years	1,132,263
Estimated Fair Value, Due after ten years	811,530
Estimated Fair Value due, Total	3,804,524
Estimated Fair Value	6,360,747
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,500,052
Estimated Fair Value	1,520,676
Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	47,645
Estimated Fair Value	47,618
Other asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,009,708
Estimated Fair Value	$ 987,929