SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Impact of Adoption (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Debt securities held to maturity, allowance for credit losses	$ 50
Allowance for credit loss	373,000	$ 365,000	$ 196,000	$ 196,000	$ 194,000	$ 188,000	$ 180,000
Allowance for credit losses on off-balance sheet credit exposures			3,000
As Reported Under ASC 326 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			301,000
Allowance for credit losses on off-balance sheet credit exposures			13,000
Impact of ASC 326 Adoption [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			105,000
Allowance for credit losses on off-balance sheet credit exposures			10,000
States of the U.S. and Political Subdivisions (municipals) [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Debt securities held to maturity, allowance for credit losses	50		0
States of the U.S. and Political Subdivisions (municipals) [Member] | As Reported Under ASC 326 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Debt securities held to maturity, allowance for credit losses			0
States of the U.S. and Political Subdivisions (municipals) [Member] | Impact of ASC 326 Adoption [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Debt securities held to maturity, allowance for credit losses			0
Commercial Real Estate [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss	184,000	163,000	60,000	60,000
Commercial Real Estate [Member] | As Reported Under ASC 326 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			138,000
Commercial Real Estate [Member] | Impact of ASC 326 Adoption [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			78,000
Commercial and Industrial [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss	93,000	98,000	53,000	53,000
Commercial and Industrial [Member] | As Reported Under ASC 326 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			65,000
Commercial and Industrial [Member] | Impact of ASC 326 Adoption [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			12,000
Commercial Leases [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss	17,000	17,000	11,000	11,000
Commercial Leases [Member] | As Reported Under ASC 326 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			11,000
Commercial Leases [Member] | Impact of ASC 326 Adoption [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			0
Other [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss	1,000	1,000	9,000	9,000
Other [Member] | As Reported Under ASC 326 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			0
Other [Member] | Impact of ASC 326 Adoption [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			(9,000)
Direct Installment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss	24,000	25,000	13,000	13,000
Direct Installment [Member] | As Reported Under ASC 326 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			24,000
Direct Installment [Member] | Impact of ASC 326 Adoption [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			11,000
Residential Mortgages [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss	31,000	33,000	22,000	22,000
Residential Mortgages [Member] | As Reported Under ASC 326 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			32,000
Residential Mortgages [Member] | Impact of ASC 326 Adoption [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			10,000
Indirect Installment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss	12,000	17,000	19,000	19,000
Indirect Installment [Member] | As Reported Under ASC 326 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			21,000
Indirect Installment [Member] | Impact of ASC 326 Adoption [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			2,000
Consumer Lines of Credit [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss	$ 11,000	$ 11,000	9,000	$ 9,000
Consumer Lines of Credit [Member] | As Reported Under ASC 326 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			10,000
Consumer Lines of Credit [Member] | Impact of ASC 326 Adoption [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit loss			$ 1,000