Investments - Equity Securities At Fair Value (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Equity securities, at fair value	$ 19,102	$ 22,163
Public Equity [Member]
Percentage of investment in equity securities	15.00%	11.00%
Private Equity [Member]
Percentage of investment in equity securities	85.00%	89.00%
Public And Private Equity [Member]
Percentage of investment in equity securities	100.00%	100.00%
Corporate [Member]
Percentage of investment in equity securities	82.00%	79.00%
Real Estate [Member]
Percentage of investment in equity securities	18.00%	21.00%
Corporate And Real Estate [Member]
Percentage of investment in equity securities	100.00%	100.00%