Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Equity securities, cost	$ 2,816,572	$ 2,740,984
Debt securities, amortized cost	$ 12,927,103	$ 13,529,923
Common stock,Shares authorized	22,000,000	22,000,000
Common stock,Shares issued	17,459,961	17,459,961
Treasury stock, shares	2,049,797	1,915,884