Parent Company Financial Statements (Statement of Income) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Expenses:
Interest on junior subordinated debentures									$ 613	$ 491	$ 871
Legal and professional fees									2,294	2,579	2,741
Acquisition related expenses									0	0	989
Change in fair value of contingent consideration									(643)	(898)	41
Income before income taxes	$ 21,145	$ 19,288	$ 19,704	$ 17,503	$ 18,035	$ 18,190	$ 16,210	$ 16,419	77,640	68,854	64,343
Income tax benefit (expense)	(13,163)	(6,326)	(6,505)	(5,721)	(5,873)	(5,863)	(5,153)	(5,484)	(31,715)	(22,373)	(20,878)
Net income	$ 7,982	$ 12,962	$ 13,199	$ 11,782	$ 12,162	$ 12,327	$ 11,057	$ 10,935	45,925	46,481	43,465
Parent Company [Member]
Income:
Dividends from subsidiaries									27,900	24,542	23,399
Other income									18	15	13
Total income									27,918	24,557	23,412
Expenses:
Interest on junior subordinated debentures									613	491	871
Legal and professional fees									169	98	134
Acquisition related expenses									0	0	308
Change in fair value of contingent consideration									(643)	(898)	41
Other									298	282	254
Total expenses									437	(27)	1,608
Income before income taxes									27,481	24,584	21,804
Income tax benefit (expense)									340	(15)	557
Income before equity in undistributed earnings of subsidiaries									27,821	24,569	22,361
Equity in undistributed earnings of subsidiaries									18,104	21,912	21,104
Net income									$ 45,925	$ 46,481	$ 43,465