ALLOWANCE FOR CREDIT LOSSES AND CREDIT QUALITY INFORMATION - Schedule of Allowance for Loan Losses and Loan Balances (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Allowance for credit losses
Beginning balance	$ 132,418	$ 232,192	$ 228,804	$ 47,576
Charge-offs	(10,861)	(2,632)	(23,415)	(9,911)
Recoveries	4,624	450	8,515	5,090
Provision (credit)	(21,310)	2,716	(109,033)	154,116
Ending balance	104,871	232,726	104,871	232,726
Loans evaluated on an individual basis	2,255	16	2,255	16
Loans evaluated on a collective basis	102,616	232,710	102,616	232,710
Loans evaluated on an individual basis	39,711	32,849	39,711	32,849
Loans evaluated on a collective basis	7,971,988	9,289,710	7,971,988	9,289,710
Ending balance	8,011,699	9,322,559	8,011,699	9,322,559	$ 9,014,677
Reverse mortgage, fair value	7,500	12,500	7,500	12,500	10,100
Loans purchased with credit deterioration				100
Impact of adoption ASC 326
Allowance for credit losses
Beginning balance				35,855
Commercial | Commercial and industrial
Allowance for credit losses
Beginning balance	87,203	144,225	150,875	22,849
Charge-offs	(7,612)	(2,254)	(19,176)	(7,390)
Recoveries	4,031	223	6,550	4,038
Provision (credit)	(17,804)	6,335	(72,431)	109,285
Ending balance	65,818	148,529	65,818	148,529
Loans evaluated on an individual basis	2,247	16	2,247	16
Loans evaluated on a collective basis	63,571	148,513	63,571	148,513
Loans evaluated on an individual basis	24,375	14,946	24,375	14,946
Loans evaluated on a collective basis	2,204,139	3,115,408	2,204,139	3,115,408
Ending balance	2,228,514	3,130,354	2,228,514	3,130,354	2,949,303
Commercial | Commercial and industrial | Impact of adoption ASC 326
Allowance for credit losses
Beginning balance				19,747
Commercial | Owner-occupied commercial
Allowance for credit losses
Beginning balance	6,181	8,956	9,615	4,616
Charge-offs	(38)	0	(83)	(336)
Recoveries	41	8	146	133
Provision (credit)	(1,260)	1,420	(4,754)	7,443
Ending balance	4,924	10,384	4,924	10,384
Loans evaluated on an individual basis	0	0	0	0
Loans evaluated on a collective basis	4,924	10,384	4,924	10,384
Loans evaluated on an individual basis	2,071	5,132	2,071	5,132
Loans evaluated on a collective basis	1,337,589	1,339,362	1,337,589	1,339,362
Ending balance	1,339,660	1,344,494	1,339,660	1,344,494	1,332,727
Commercial | Owner-occupied commercial | Impact of adoption ASC 326
Allowance for credit losses
Beginning balance				(1,472)
Commercial | Commercial mortgages
Allowance for credit losses
Beginning balance	16,099	38,397	31,071	7,452
Charge-offs	0	(4)	0	(55)
Recoveries	198	6	242	38
Provision (credit)	(3,054)	(4,140)	(18,070)	25,162
Ending balance	13,243	34,259	13,243	34,259
Loans evaluated on an individual basis	8	0	8	0
Loans evaluated on a collective basis	13,235	34,259	13,235	34,259
Loans evaluated on an individual basis	2,708	4,878	2,708	4,878
Loans evaluated on a collective basis	1,985,276	2,162,630	1,985,276	2,162,630
Ending balance	1,987,984	2,167,508	1,987,984	2,167,508	2,086,062
Commercial | Commercial mortgages | Impact of adoption ASC 326
Allowance for credit losses
Beginning balance				1,662
Commercial | Construction
Allowance for credit losses
Beginning balance	3,512	10,126	12,190	3,891
Charge-offs	(2,473)	0	(2,473)	0
Recoveries	0	0	0	5
Provision (credit)	1,068	104	(7,610)	5,653
Ending balance	2,107	10,230	2,107	10,230
Loans evaluated on an individual basis	0	0	0	0
Loans evaluated on a collective basis	2,107	10,230	2,107	10,230
Loans evaluated on an individual basis	2,184	83	2,184	83
Loans evaluated on a collective basis	760,462	666,234	760,462	666,234
Ending balance	762,646	666,317	762,646	666,317	716,275
Commercial | Construction | Impact of adoption ASC 326
Allowance for credit losses
Beginning balance				681
Residential
Allowance for credit losses
Beginning balance	3,293	9,171	6,893	1,381
Charge-offs	0	0	0	(175)
Recoveries	34	26	629	141
Provision (credit)	(326)	(743)	(4,521)	(415)
Ending balance	3,001	8,454	3,001	8,454
Loans evaluated on an individual basis	0	0	0	0
Loans evaluated on a collective basis	3,001	8,454	3,001	8,454
Loans evaluated on an individual basis	5,877	5,307	5,877	5,307
Loans evaluated on a collective basis	568,804	839,688	568,804	839,688
Ending balance	574,681	844,995	574,681	844,995	764,393
Residential | Impact of adoption ASC 326
Allowance for credit losses
Beginning balance				7,522
Consumer
Allowance for credit losses
Beginning balance	16,130	21,317	18,160	7,387
Charge-offs	(738)	(374)	(1,683)	(1,955)
Recoveries	320	187	948	735
Provision (credit)	66	(260)	(1,647)	6,988
Ending balance	15,778	20,870	15,778	20,870
Loans evaluated on an individual basis	0	0	0	0
Loans evaluated on a collective basis	15,778	20,870	15,778	20,870
Loans evaluated on an individual basis	2,496	2,503	2,496	2,503
Loans evaluated on a collective basis	1,115,718	1,166,388	1,115,718	1,166,388
Ending balance	$ 1,118,214	$ 1,168,891	$ 1,118,214	1,168,891	$ 1,165,917
Consumer | Impact of adoption ASC 326
Allowance for credit losses
Beginning balance				$ 7,715