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	Sep. 30, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, Due in one year or less	$ 769,507
Amortized Cost, Due after one year through five years	1,202,643
Amortized Cost, Due after five years through ten years	443,917
Amortized Cost, Due after ten years	225,990
Amortized Cost, Total	2,642,057
Estimated Fair Value, Due in one year or less	769,767
Estimated Fair Value, Due after one year through five years	1,221,570
Estimated Fair Value, Due after five years through ten years	471,856
Estimated Fair Value, Due after ten years	259,122
Estimated Fair Value due, Total	$ 2,722,315