Parent Company Financial Statements (Statements of Cash Flows) (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
Cash flows from operating activities:
Net income	$ 7,982	$ 12,962	$ 13,199	$ 11,782	$ 12,162	$ 12,327	$ 11,057	$ 10,935	$ 45,925	$ 46,481	$ 43,465
Adjustments to reconcile net income to net cash provided by operating activities:
(Increase) decrease in other assets									8,794	(984)	4,165
Change in fair value of contingent consideration									(643)	(898)	41
Tax benefit from stock option exercises and other equity instruments									508	1,016	694
Net cash provided by operating activities									59,385	59,749	52,260
Cash flows from investing activities:
Cash used in business combination, net of cash acquired									0	0	(1,671)
Net cash used in investing activities									(179,706)	(625,377)	(165,885)
Cash flows from financing activities:
Proceeds from stock option exercises and issuance of other equity instruments									366	977	1,563
Cash dividends paid									(26,300)	(24,637)	(22,770)
Net cash provided by financing activities									95,447	575,794	130,906
Net (decrease) increase in cash and cash equivalents									(24,874)	10,166	17,281
Cash and cash equivalents at beginning of year				107,797				97,631	107,797	97,631	80,350
Cash and cash equivalents at end of year	82,923				107,797				82,923	107,797	97,631
Parent Company [Member]
Cash flows from operating activities:
Net income									45,925	46,481	43,465
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed earnings of subsidiary									(18,104)	(21,912)	(21,104)
(Increase) decrease in dividend receivable									(1,252)	(1,436)	19
(Increase) decrease in other assets									72	45	(67)
Increase (decrease) in accrued expenses and other liabilities									22	(12)	2,425
Change in fair value of contingent consideration									(643)	(898)	41
Tax benefit from stock option exercises and other equity instruments									508	1,016	694
Other, net									(673)	(1,051)	(3,363)
Net cash provided by operating activities									25,855	22,233	22,110
Cash flows from investing activities:
Cash used in business combination, net of cash acquired									0	0	(1,671)
Net cash used in investing activities									0	0	(1,671)
Cash flows from financing activities:
Proceeds from stock option exercises and issuance of other equity instruments									366	978	1,563
Cash dividends paid									(26,300)	(24,637)	(22,770)
Net cash provided by financing activities									(25,934)	(23,659)	(21,207)
Net (decrease) increase in cash and cash equivalents									(79)	(1,426)	(768)
Cash and cash equivalents at beginning of year				$ 1,743				$ 3,169	1,743	3,169	3,937
Cash and cash equivalents at end of year	$ 1,664				$ 1,743				$ 1,664	$ 1,743	$ 3,169