Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Equity securities, cost	$ 3,721,932	$ 3,170,673
Debt securities, amortized cost	$ 12,065,159	$ 12,536,772
Common stock, Shares authorized	22,000,000	22,000,000
Common stock, Shares issued	17,459,961	17,459,961
Treasury stock, shares	2,097,820	2,069,461