Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Statement Of Financial Position [Abstract]
Equity securities, cost	$ 2,617,050	$ 2,051,996
Debt securities, amortized cost	14,514,691	14,898,522
Allowance for credit losses	$ 607	$ 2,579
Common stock, Shares authorized	22,000,000	22,000,000
Common stock, Shares issued	17,459,961	17,459,961
Treasury stock, shares	3,519,292	3,418,781