Debt and Equity Securities (Investments in Equity Securities) (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt And Equity Securities F V N I [Line Items]
Equity securities, Cost	$ 330,555	$ 376,320
Equity securities, Estimated fair value	419,606	464,126
Preferred stocks
Debt And Equity Securities F V N I [Line Items]
Equity securities, Cost	21,803	22,163
Equity securities, Estimated fair value	20,920	19,479
Common stocks
Debt And Equity Securities F V N I [Line Items]
Equity securities, Cost	308,752	354,157
Equity securities, Estimated fair value	$ 398,686	$ 444,647