Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Equity securities, cost	$ 2,740,984	$ 2,366,035
Debt securities, amortized cost	$ 13,529,923	$ 14,364,430
Common stock, shares authorized	22,000,000	22,000,000
Common stock, shares issued	17,459,961	17,459,961
Treasury stock, shares	1,915,884	1,405,638