Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Equity securities, cost	$ 3,170,673	$ 2,816,572
Debt securities, amortized cost	$ 12,536,772	$ 12,927,103
Common stock, Shares authorized	22,000,000	22,000,000
Common stock, Shares issued	17,459,961	17,459,961
Treasury stock, shares	2,069,461	2,049,797