QUARTERLY FINANCIAL INFORMATION OF REGISTRANT (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2020		Sep. 30, 2020		Jun. 30, 2020		Mar. 31, 2020		Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 56,904		$ 55,677		$ 58,495		$ 60,752		$ 60,533		$ 60,555		$ 60,672		$ 59,027		$ 231,828	$ 240,787	$ 229,165
Interest expense	8,197		9,091		11,224		16,051		17,357		18,182		17,541		17,902		44,563	70,982	57,101
Net interest income	48,707		46,586		47,271		44,701		43,176		42,373		43,131		41,125		187,265	169,805	172,064
Provision for credit losses	(5,545)	[1]	(4,746)	[1]	5,245	[1]	25,247	[1]	2,508	[1]	1,005	[1]	2,506	[1]	(918)	[1]	20,201	5,101	8,437
Net interest income after provision for credit losses	54,252		51,332		42,026		19,454		40,668		41,368		40,625		42,043		167,064	164,704	163,627
Noninterest income excluding net gain (loss) on sale of securities	10,924		11,063		9,531		9,957		10,423		11,069		10,838		9,282
Net gain (loss) on sale of securities available for sale	(24)		78		2,662		5,541		42		42		416		256		8,257	756	(1,839)
Noninterest expense	31,315		31,616		29,856		30,520		30,944		29,026		29,700		29,627		123,307	119,297	120,099
Income before income tax expense	33,837		30,857		24,363		4,432		20,189		23,453		22,179		21,954		93,489	87,775	84,301
Income tax expense	4,265		3,783		2,809		479		2,854		3,661		3,569		3,137		11,336	13,221	10,163
Net income	$ 29,572		$ 27,074		$ 21,554		$ 3,953		$ 17,335		$ 19,792		$ 18,610		$ 18,817		$ 82,153	$ 74,554	$ 74,138
Earnings (loss) per common share
Earnings (loss) per common share - basic (in dollars per share)	$ 0.89		$ 0.82		$ 0.65		$ 0.12		$ 0.51		$ 0.59		$ 0.55		$ 0.56		$ 2.47	$ 2.21	$ 2.12
Earnings (loss) per common share - diluted (in dollars per share)	0.89		0.82		0.65		0.12		0.51		0.58		0.55		0.56		2.47	2.20	2.11
Cash dividends paid per common share	0.37		0.31		0.31		0.31		0.34		0.31		0.31		0.30		$ 1.30	$ 1.26	$ 1.20
Book value per common share	$ 26.56		$ 25.37		$ 24.75		$ 24.11		$ 23.79		$ 23.98		$ 23.34		$ 22.48

[1] Upon adoption of CECL on January 1, 2020, the provision for credit losses is the sum of the provision for loan losses and the provision for off-balance-sheet credit exposures. Prior to the adoption of CECL, the provision for off-balance-sheet credit exposures was included in other noninterest expense.