SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Interest income	$ 87,910,000	$ 82,875,000	$ 79,167,000	$ 73,749,000	$ 72,876,000	$ 71,778,000	$ 68,133,000	$ 64,407,000	$ 323,701,000	$ 277,194,000	$ 246,637,000
Interest expense	7,618,000	6,641,000	5,999,000	5,278,000	5,044,000	4,705,000	4,378,000	4,207,000	25,536,000	18,334,000	18,793,000
Net interest income	80,292,000	76,234,000	73,168,000	68,471,000	67,832,000	67,073,000	63,755,000	60,200,000	298,165,000	258,860,000	227,844,000
Provision (benefit)	1,200,000	1,075,000	2,000,000	500,000	1,300,000	0	1,050,000	600,000	4,775,000	2,950,000	6,075,000
Total noninterest income	23,491,000	23,264,000	21,887,000	19,863,000	21,914,000	20,770,000	21,398,000	18,912,000	88,505,000	82,994,000	82,428,000
Total noninterest expenses	64,391,000	55,439,000	52,688,000	53,451,000	51,467,000	51,310,000	52,809,000	48,773,000	225,969,000	204,359,000	192,122,000
Provision for income taxes	8,258,000	9,969,000	9,249,000	6,828,000	14,915,000	12,681,000	10,731,000	9,014,000	34,304,000	47,341,000	35,427,000
Net Income	$ 29,934,000	$ 33,015,000	$ 31,118,000	$ 27,555,000	$ 22,064,000	$ 23,852,000	$ 20,563,000	$ 20,725,000	$ 121,622,000	$ 87,204,000	$ 76,648,000
Basic earnings per share (in dollars per share)	$ 1.08	$ 1.20	$ 1.13	$ 1.00	$ 0.80	$ 0.87	$ 0.75	$ 0.77	$ 4.41	$ 3.19	$ 2.90
Diluted earnings per share (in dollars per share)	$ 1.07	$ 1.20	$ 1.13	$ 1.00	$ 0.80	$ 0.87	$ 0.75	$ 0.76	$ 4.40	$ 3.19	$ 2.90
Weighted average common shares (basic) (in shares)	27,815,437	27,537,841	27,526,653	27,486,573	27,445,739	27,436,792	27,257,799	27,029,640	27,592,380	27,294,028	26,404,071
Common share equivalents (in shares)	58,576	63,499	54,525	67,381	77,615	76,307	74,497	81,283	61,428	78,076	51,847
Weighted average common shares (diluted) (in shares)	27,874,013	27,601,340	27,581,178	27,553,954	27,523,354	27,513,099	27,332,296	27,110,923	27,653,808	27,372,104	26,455,918
Acquisition Related Costs	$ 8,046,000	$ 2,688,000	$ 434,000	$ 0		$ 0	$ 2,909,000	$ 484,000	$ 11,168,000	$ 3,393,000	$ 5,455,000
Tax Expense related to a change in tax law		0	0	0		0	0	0	0	466,000	$ 0
Total unusual or infrequent expense items	8,046,000	$ 2,688,000	$ 434,000	$ 0	$ 2,361,000	$ 0	$ 2,909,000	$ 484,000
Net Deferred Tax Asset [Member]
Tax Expense related to a change in tax law	0				1,895,000				$ 71,000	1,900,000
Investments in Low Income Housing Projects [Member]
Tax Expense related to a change in tax law	$ 0				$ 466,000					$ 466,000