SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								6 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	Jun. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Interest income	$ 96,805	$ 97,919	$ 99,965	$ 107,380	$ 113,703	$ 119,624	$ 122,144	$ 91,543		$ 402,069	$ 447,014	$ 323,701
Interest expense	5,362	7,036	8,867	13,076	13,710	15,026	16,125	9,018		34,341	53,879	25,536
Net interest income	91,443	90,883	91,098	94,304	99,993	104,598	106,019	82,525		367,728	393,135	298,165
Provision for Loan and Lease Losses	0	7,500	20,000	25,000	4,000	0	1,000	1,000	$ 4,775	52,500	6,000	4,775
Total noninterest income	27,468	29,347	28,190	26,435	33,297	31,816	28,648	21,533		111,440	115,294	88,505
Total noninterest expenses	73,727	66,658	66,607	66,840	67,445	67,533	93,032	56,311		273,832	284,321	225,969
Current Federal, State and Local, Tax Expense (Benefit)	10,543
Provision for income taxes		11,199	7,779	2,148	14,368	17,036	10,007	11,522		31,669	52,933	34,304
Net Income	$ 34,641	$ 34,873	$ 24,902	$ 26,751	$ 47,477	$ 51,845	$ 30,628	$ 35,225		$ 121,167	$ 165,175	$ 121,622
Basic earnings per share (in dollars per share)	$ 1.05	$ 1.06	$ 0.76	$ 0.78	$ 1.38	$ 1.51	$ 0.89	$ 1.25		$ 3.64	$ 5.03	$ 4.41
Diluted earnings per share (in dollars per share)	$ 1.05	$ 1.06	$ 0.76	$ 0.78	$ 1.38	$ 1.51	$ 0.89	$ 1.25		$ 3.64	$ 5.03	$ 4.40
Weighted average common shares (basic) (in shares)	32,964,090	32,951,918	32,944,761	34,184,431	34,374,953	34,361,176	34,313,492	28,106,184		33,259,643	32,810,433	27,592,380
Common share equivalents (in shares)	26,348	24,758	28,098	36,827	46,245	39,390	41,878	54,466		25,646	45,801	61,428
Weighted average common shares (diluted) (in shares)	32,990,438	32,976,676	32,972,859	34,221,258	34,421,198	34,400,566	34,355,370	28,160,650		33,285,289	32,856,234	27,653,808
Gain (Loss) on Sales of Loans, Net		$ 0								$ 30,100	$ 13,200	$ 3,600
Total Unusual or infrequent Income Items	$ 0	0	$ 0	$ 0	$ 0	$ 951	$ 0	$ 0
Acquisition Related Costs	0	0		0		705	24,696	1,032		0	26,433	11,168
Adjustment for tax effect of previously incurred M&A expense				0
Total unusual or infrequent expense items	$ 0	684	$ 0	$ 0	$ 0	$ 705	$ 24,696	$ 1,682
Loss on Contract Termination		$ 684								$ 684	$ 0	$ 0