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 Millions	Mar. 31, 2020 USD ($)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, Due in one year or less	$ 683
Amortized Cost, Due after one year through five years	1,083
Amortized Cost, Due after five years through ten years	447
Amortized Cost, Due after ten years	145
Amortized Cost, Total	2,359
Estimated Fair Value, Due in one year or less	683
Estimated Fair Value, Due after one year through five years	1,062
Estimated Fair Value, Due after five years through ten years	462
Estimated Fair Value, Due after ten years	161
Estimated Fair Value due, Total	$ 2,368