SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Effect of New Financial Accounting Standards) (Details) - USD ($) $ 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	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use assets	$ 21,557				$ 23,200				$ 21,557	$ 23,200
Present value of lease liabilities	25,337				24,617				25,337	24,617
Net deferred tax asset	8,122				13,600				8,122	13,600
Adjustment to retained earnings	791,630				702,502				791,630	702,502
Increase in allowance	131,606				70,395				131,606	70,395	$ 61,963	$ 55,686
Reclassification to debt securities available for sale	6,108,346				3,294,361				6,108,346	3,294,361
Reclassification from held to maturity securities	(88,890)				(91,324)				(88,890)	(91,324)
Income taxes (includes $2,376, $1,890, and $165 of income tax expense reclassified from accumulated other comprehensive income (loss) for the years ended December 31, 2020, 2019, and 2018, respectively)	9,046	$ 13,681	$ 7,417	$ 5,909	5,155	$ 7,941	$ 13,584	$ 8,310	36,053	34,990	$ 28,215
Off-Balance Sheet, Credit Loss, Liability	15,280				248				15,280	248
Debt Securities, Held-to-maturity, Allowance for Credit Loss	$ 51				0				$ 51	0
Impact of ASU 2016-13 adoption on January 1, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment to retained earnings					14,900					14,900
Increase in allowance					12,071					12,071
Income taxes (includes $2,376, $1,890, and $165 of income tax expense reclassified from accumulated other comprehensive income (loss) for the years ended December 31, 2020, 2019, and 2018, respectively)										5,000
Off-Balance Sheet, Credit Loss, Liability					13,604					13,604
Debt Securities, Held-to-maturity, Allowance for Credit Loss					158					158
Impact of ASU 2016-13 adoption on January 1, 2020 | Held-to-maturity Securities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment to retained earnings					118					118
Income taxes (includes $2,376, $1,890, and $165 of income tax expense reclassified from accumulated other comprehensive income (loss) for the years ended December 31, 2020, 2019, and 2018, respectively)										40
Loans | Impact of ASU 2016-13 adoption on January 1, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment to retained earnings					4,600					4,600
Increase in allowance					12,100					12,100
Income taxes (includes $2,376, $1,890, and $165 of income tax expense reclassified from accumulated other comprehensive income (loss) for the years ended December 31, 2020, 2019, and 2018, respectively)										1,500
Loans | Impact of ASU 2016-13 adoption on January 1, 2020 | Financial Asset Acquired with Credit Deterioration
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in allowance					6,000					6,000
Unfunded Loan Commitment | Impact of ASU 2016-13 adoption on January 1, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment to retained earnings					10,200					10,200
Income taxes (includes $2,376, $1,890, and $165 of income tax expense reclassified from accumulated other comprehensive income (loss) for the years ended December 31, 2020, 2019, and 2018, respectively)										3,400
Off-Balance Sheet, Credit Loss, Liability					$ 13,600					$ 13,600