Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt securities, available-for-sale, cost (in dollars)	$ 604,999	$ 597,784
Equity securities, available for sale, cost (in dollars)	30,253	31,449
Other investments, available-for-sale, cost (in dollars)	639,015	632,996
Unamortized debt issuance cost (in dollars)	$ 949	$ 1,001
Common stock, par value (in dollars per share)	$ 0.18	$ 0.18
Common stock, authorized shares	33,333,333	33,333,333
Common stock, issued shares	20,872,831	20,872,831
Treasury stock, shares	2,703,803	2,260,849
Other Investments [Member]
Other investments, available-for-sale, cost (in dollars)	$ 3,763	$ 3,763