LOANS AND ALLOWANCE FOR CREDIT LOSSES Credit Risk Profile by Creditworthiness Category (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss	$ 8,982,336	$ 9,502,428	$ 9,502,428
Consumer
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			942,179
Consumer | Home equity
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss	349,274	378,793	380,591
Consumer | Auto And Other Receivable
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			561,588
Commercial and industrial loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss	1,819,175	1,522,059	1,840,508
Residential mortgages
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			2,685,472
Residential mortgages | 1 to 4 Family
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			2,677,353
Residential mortgages | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			8,119
Residential mortgages | Residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss	2,270,458	$ 2,853,385	2,853,385
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			4,034,269
Commercial real estate | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			429,806
Business Activities Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			7,166,497
Business Activities Loans | Consumer
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			778,466
Business Activities Loans | Consumer | Home equity
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			273,867
Business Activities Loans | Consumer | Auto And Other Receivable
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			504,599
Business Activities Loans | Commercial and industrial loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			1,442,617
Business Activities Loans | Residential mortgages
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			2,148,458
Business Activities Loans | Residential mortgages | 1 to 4 Family
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			2,143,817
Business Activities Loans | Residential mortgages | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			4,641
Business Activities Loans | Commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			2,796,956
Business Activities Loans | Commercial real estate | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			382,014
Business Activities Loans | Commercial real estate | Residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			2,414,942
Acquired Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			2,335,931
Acquired Loans | Consumer
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			163,713
Acquired Loans | Consumer | Home equity
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			106,724
Acquired Loans | Consumer | Auto And Other Receivable
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			56,989
Acquired Loans | Commercial and industrial loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			397,891
Acquired Loans | Residential mortgages
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			537,014
Acquired Loans | Residential mortgages | 1 to 4 Family
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			533,536
Acquired Loans | Residential mortgages | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			3,478
Acquired Loans | Commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			1,237,313
Acquired Loans | Commercial real estate | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			47,792
Acquired Loans | Commercial real estate | Residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			1,189,521
Pass | Commercial and industrial loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss	1,763,426
Pass | Residential mortgages | Residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss	2,258,162
Pass | Business Activities Loans | Commercial and industrial loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			1,366,342
Pass | Business Activities Loans | Residential mortgages
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			2,144,394
Pass | Business Activities Loans | Residential mortgages | 1 to 4 Family
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			2,139,753
Pass | Business Activities Loans | Residential mortgages | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			4,641
Pass | Business Activities Loans | Commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			2,736,389
Pass | Business Activities Loans | Commercial real estate | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			382,014
Pass | Business Activities Loans | Commercial real estate | Residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			2,354,375
Pass | Acquired Loans | Commercial and industrial loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			373,744
Pass | Acquired Loans | Residential mortgages
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			531,760
Pass | Acquired Loans | Residential mortgages | 1 to 4 Family
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			528,282
Pass | Acquired Loans | Residential mortgages | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			3,478
Pass | Acquired Loans | Commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			1,176,729
Pass | Acquired Loans | Commercial real estate | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			46,396
Pass | Acquired Loans | Commercial real estate | Residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			1,130,333
Special mention
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			75,284
Special mention | Commercial and industrial loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss	29,368
Special mention | Residential mortgages | Residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss	545
Special mention | Business Activities Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			63,943
Special mention | Business Activities Loans | Commercial and industrial loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			50,072
Special mention | Business Activities Loans | Residential mortgages
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			714
Special mention | Business Activities Loans | Residential mortgages | 1 to 4 Family
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			714
Special mention | Business Activities Loans | Residential mortgages | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			0
Special mention | Business Activities Loans | Commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			12,167
Special mention | Business Activities Loans | Commercial real estate | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			0
Special mention | Business Activities Loans | Commercial real estate | Residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			12,167
Special mention | Acquired Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			11,341
Special mention | Acquired Loans | Commercial and industrial loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			4,404
Special mention | Acquired Loans | Residential mortgages
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			592
Special mention | Acquired Loans | Residential mortgages | 1 to 4 Family
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			592
Special mention | Acquired Loans | Residential mortgages | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			0
Special mention | Acquired Loans | Commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			5,993
Special mention | Acquired Loans | Commercial real estate | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			0
Special mention | Acquired Loans | Commercial real estate | Residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			5,993
Substandard
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			122,424
Substandard | Commercial and industrial loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss	25,979
Substandard | Residential mortgages | Residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss	11,751
Substandard | Business Activities Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			49,293
Substandard | Business Activities Loans | Commercial and industrial loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			24,112
Substandard | Business Activities Loans | Residential mortgages
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			3,350
Substandard | Business Activities Loans | Residential mortgages | 1 to 4 Family
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			3,350
Substandard | Business Activities Loans | Residential mortgages | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			0
Substandard | Business Activities Loans | Commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			48,400
Substandard | Business Activities Loans | Commercial real estate | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			0
Substandard | Business Activities Loans | Commercial real estate | Residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			48,400
Substandard | Acquired Loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			73,131
Substandard | Acquired Loans | Commercial and industrial loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			19,743
Substandard | Acquired Loans | Residential mortgages
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			4,662
Substandard | Acquired Loans | Residential mortgages | 1 to 4 Family
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			4,662
Substandard | Acquired Loans | Residential mortgages | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			0
Substandard | Acquired Loans | Commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			54,591
Substandard | Acquired Loans | Commercial real estate | Construction
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			1,396
Substandard | Acquired Loans | Commercial real estate | Residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			53,195
Doubtful | Commercial and industrial loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss	402
Doubtful | Business Activities Loans | Commercial and industrial loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			2,091
Performing | Consumer | Home equity
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss	345,646
Performing | Business Activities Loans | Consumer
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			774,643
Performing | Business Activities Loans | Consumer | Home equity
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			272,772
Performing | Business Activities Loans | Consumer | Auto And Other Receivable
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			501,871
Performing | Acquired Loans | Consumer
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			162,731
Performing | Acquired Loans | Consumer | Home equity
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			106,007
Performing | Acquired Loans | Consumer | Auto And Other Receivable
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			56,724
Nonperforming | Consumer | Home equity
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss	$ 3,628
Nonperforming | Business Activities Loans | Consumer
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			3,823
Nonperforming | Business Activities Loans | Consumer | Home equity
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			1,095
Nonperforming | Business Activities Loans | Consumer | Auto And Other Receivable
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			2,728
Nonperforming | Acquired Loans | Consumer
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			982
Nonperforming | Acquired Loans | Consumer | Home equity
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			717
Nonperforming | Acquired Loans | Consumer | Auto And Other Receivable
Financing Receivable, Allowance for Credit Loss [Line Items]
Financing receivable, before allowance for credit loss			$ 265