ACQUISITIONS (Narrative) (Details) - USD ($)				12 Months Ended
	Dec. 04, 2020	Nov. 30, 2019	May 10, 2019	Dec. 31, 2020	Dec. 31, 2019
Business Acquisition [Line Items]
Assets				$ 17,908,339,000	$ 13,209,597,000
Held to maturity				10,023,051,000	8,367,917,000
Deposits				14,979,905,000	$ 11,044,331,000
Arizona Bank & Trust
Business Acquisition [Line Items]
Assets	$ 419,700,000
Held to maturity	150,700,000
Deposits	415,500,000
AIM Bancshares, Inc.
Business Acquisition [Line Items]
Assets	1,970,000,000
Held to maturity	1,090,000,000.00
Deposits	$ 1,670,000,000
Rockford Bank and Trust Company
Business Acquisition [Line Items]
Assets		$ 495,700,000
Held to maturity		354,000,000.0
Deposits		430,300,000
Cash payments to acquire business		$ 46,600,000
Blue Valley Ban Corp.
Business Acquisition [Line Items]
Assets			$ 766,200,000
Held to maturity			542,000,000.0
Deposits			$ 617,100,000
AIM Bancshares, Inc.
Business Acquisition [Line Items]
Number of shares issued in exchange per share (in shares)	$ 207
Cash issued in exchange per share	$ 1,887.16
Common stock price (in dollars per share)	$ 41.89
Total consideration paid	$ 264,477,000
Common stock consideration	217,200,000
Cash payments to acquire business	47,275,000			47,300,000
Merger related expenses				$ 2,500,000
Loans held-to-maturity acquired	$ 1,087,041,000
Business acquisition, number of shares issued (in shares)	5,185,045
AIM Bancshares, Inc. | Holders Of In-The-Money Options
Business Acquisition [Line Items]
Common stock consideration	$ 4,900,000
Blue Valley Ban Corp.
Business Acquisition [Line Items]
Common stock price (in dollars per share)			$ 44.78
Total consideration paid			$ 92,300,000
Business acquisition, number of shares issued (in shares)			2,060,258
Outstanding debt extinguished			$ 6,900,000
Blue Valley Ban Corp. | Trust preferred securities
Business Acquisition [Line Items]
Trust preferred securities			$ 16,100,000