Debt and Equity Securities (Debt Securities in Unrealized Loss Position) (Narrative) (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule Of Debt and Equity Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 2,924,554	$ 615,839
Bank Loans
Schedule Of Debt and Equity Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	32,900
Emerging Market Securities
Schedule Of Debt and Equity Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	15,400
Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	54,983
Non-Investment Grade | Bank Loans
Schedule Of Debt and Equity Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	31,200
Non-Investment Grade | High Yield Corporate Debt Securities
Schedule Of Debt and Equity Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	21,500
Non-Investment Grade | Emerging Market Securities
Schedule Of Debt and Equity Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 2,300