Schedule II - Condensed Financial Information of Registrant (Parent Company Only) (Balance Sheet Extra) (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt securities, available-for-sale, cost (in dollars)	$ 604,999	$ 597,784
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
Parent Company [Member]
Debt securities, available-for-sale, cost (in dollars)	$ 150
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