DERIVATIVE FINANCIAL INSTRUMENTS AND HEDGING ACTIVITIES - Derivative Assets and Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	$ 3,843,761	$ 4,070,729
Estimated fair value asset (liability)	105,967	(492)
Impact of settled to market contracts	109,000
Interest rate swaps
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	9,400
Total economic hedges
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	3,800,000
Total economic hedges | Interest rate swaps
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	3,400,000
Total economic hedges | Risk participation agreements with dealer banks
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	300,000
Total economic hedges | Forward sale commitments
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	17,600
Designated as Hedging Instrument | Total economic hedges
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	3,768,671	3,901,732
Estimated fair value asset (liability)	104,469	(3,120)
Designated as Hedging Instrument | Total economic hedges | Interest rate swaps | Interest rate swap on tax advantaged economic development bond
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	$ 8,843	$ 9,390
Weighted average maturity	9 years 2 months 12 days	9 years 10 months 24 days
Weighted average rate, received	0.52%	2.08%
Weighted average rate, contract pay rate	5.09%	5.09%
Estimated fair value asset (liability)	$ (1,933)	$ (1,488)
Designated as Hedging Instrument | Total economic hedges | Interest rate swaps | Loans with commercial loan customers
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	$ 1,713,149	$ 1,669,895
Weighted average maturity	6 years	6 years 4 months 24 days
Weighted average rate, received	4.28%	4.38%
Weighted average rate, contract pay rate	1.93%	3.28%
Estimated fair value asset (liability)	$ 177,731	$ 75,326
Designated as Hedging Instrument | Total economic hedges | Reverse interest rate swaps | Loans with commercial loan customers
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	$ 1,713,149	$ 1,669,895
Weighted average maturity	6 years	6 years 4 months 24 days
Weighted average rate, received	1.93%	3.28%
Weighted average rate, contract pay rate	4.28%	4.38%
Estimated fair value asset (liability)	$ (72,386)	$ (77,051)
Designated as Hedging Instrument | Total economic hedges | Risk participation agreements with dealer banks
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	$ 315,936	$ 315,140
Weighted average maturity	7 years	7 years 6 months
Estimated fair value asset (liability)	$ 660	$ 320
Designated as Hedging Instrument | Total economic hedges | Forward sale commitments
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	$ 17,594	$ 237,412
Weighted average maturity	2 months 12 days	2 months 12 days
Estimated fair value asset (liability)	$ 397	$ (227)
Not Designated as Hedging Instrument
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	75,090	168,997
Estimated fair value asset (liability)	1,498	2,628
Not Designated as Hedging Instrument | Commitments to lend
Interest rate swap agreements and non-hedging derivative assets and liabilities
Notional amount	$ 75,090	$ 168,997
Weighted average maturity	2 months 12 days	2 months 12 days
Estimated fair value asset (liability)	$ 1,498	$ 2,628