Parent Company Financial Statements (Balance Sheet) (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets:
Other assets	$ 63,740	$ 62,484
Total assets	4,529,850	4,381,115	$ 3,771,604
Liabilities:
Junior subordinated debentures	22,681	22,681
Contingent consideration liability	1,404	2,047	2,945
Other liabilities	59,822	54,948
Total liabilities	4,116,566	3,990,311
Shareholders’ Equity:
Common stock of $.0625 par value; authorized 60,000,000 shares; issued and outstanding 17,226,508 shares in 2017 and 17,170,820 shares in 2016	1,077	1,073
Paid-in capital	117,961	115,123
Retained earnings	317,756	294,365
Accumulated other comprehensive loss	(23,510)	(19,757)
Total shareholders’ equity	413,284	390,804	$ 375,388	$ 346,279
Total liabilities and shareholders’ equity	$ 4,529,850	$ 4,381,115
Common stock, par value (in dollars per share)	$ 0.0625	$ 0.0625
Common stock, shares authorized (in shares)	60,000,000	60,000,000
Common stock, shares issued (in shares)	17,226,508	17,170,820
Common stock, shares outstanding (in shares)	17,226,508	17,170,820
Parent Company [Member]
Assets:
Cash on deposit with bank subsidiary	$ 1,664	$ 1,743
Investment in subsidiaries at equity value	434,831	413,613
Dividends receivable from subsidiaries	7,770	6,518
Other assets	259	332
Total assets	444,524	422,206
Liabilities:
Junior subordinated debentures	22,681	22,681
Dividends payable	7,087	6,628
Contingent consideration liability	1,404	2,047
Other liabilities	68	46
Total liabilities	31,240	31,402
Shareholders’ Equity:
Common stock of $.0625 par value; authorized 60,000,000 shares; issued and outstanding 17,226,508 shares in 2017 and 17,170,820 shares in 2016	1,077	1,073
Paid-in capital	117,961	115,123
Retained earnings	317,756	294,365
Accumulated other comprehensive loss	(23,510)	(19,757)
Total shareholders’ equity	413,284	390,804
Total liabilities and shareholders’ equity	$ 444,524	$ 422,206