Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Equity securities, cost	$ 1,399,141	$ 2,904,496
Debt securities, amortized cost	$ 14,012,407	$ 11,895,850
Common stock, Shares authorized	22,000,000	22,000,000
Common stock, Shares issued	17,459,961	17,459,961
Treasury stock, shares	3,054,453	2,883,452