Consolidated Statements of Changes in Shareholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning Balance at Dec. 31, 2014	$ 797,133	$ 32,774	$ 565,166	$ 231,646	$ (32,453)
Changes in Stockholders' Equity [Roll Forward]
Comprehensive income	38,208			36,859	1,349
Cash dividends declared and paid, $0.52 and $0.48 per share at June 30, 2016 and June 30, 2015, respectively.	(17,049)			(17,049)
Issuance of common stock in business acquisitions	159,904	5,183	154,721
Stock issued - stock options	1,146	88	1,058
Shares issued – directors’ stock plans	316	11	305
Shares issued – restricted stock units	(328)	53	(381)
Share-based compensation	1,461	1	1,460
Ending Balance at Jun. 30, 2015	980,791	38,110	722,329	251,456	(31,104)
Beginning Balance at Dec. 31, 2015	1,015,974	38,168	725,280	281,558	(29,032)
Changes in Stockholders' Equity [Roll Forward]
Comprehensive income	52,303			48,969	3,334
Cash dividends declared and paid, $0.52 and $0.48 per share at June 30, 2016 and June 30, 2015, respectively.	(19,942)			(19,942)
Stock issued - stock options	528	48	480
Shares issued – directors’ stock plans	125	5	120
Shares issued – restricted stock units	(469)	45	(514)
Share-based compensation	1,780	1	1,779
Ending Balance at Jun. 30, 2016	$ 1,050,299	$ 38,267	$ 727,145	$ 310,585	$ (25,698)