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) $ in Thousands	Jun. 30, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, Due in one year or less	$ 885,076
Amortized Cost, Due after one year through five years	1,094,834
Amortized Cost, Due after five years through ten years	391,425
Amortized Cost, Due after ten years	287,559
Amortized Cost, Total	2,658,894
Estimated Fair Value, Due in one year or less	885,352
Estimated Fair Value, Due after one year through five years	1,114,706
Estimated Fair Value, Due after five years through ten years	411,322
Estimated Fair Value, Due after ten years	316,218
Estimated Fair Value due, Total	$ 2,727,598