ACQUISITIONS (Details) - USD ($)	Dec. 04, 2020	Mar. 31, 2021	Dec. 31, 2020
Business Acquisition [Line Items]
Assets acquired		$ 18,244,427,000	$ 17,908,339,000
Deposits acquired		15,559,051,000	14,979,905,000
Gross loans held to maturity		$ 10,050,456,000	$ 10,023,051,000
Share price (USD per share)	$ 41.89
Johnson Bank Branches | Arizona Bank & Trust
Business Acquisition [Line Items]
Assets acquired	$ 419,700,000
Deposits acquired	415,500,000
Gross loans held to maturity	150,700,000
AIM Bancshares, Inc.
Business Acquisition [Line Items]
Assets acquired	1,970,000,000
Deposits acquired	1,670,000,000
Gross loans held to maturity	$ 1,090,000,000.00
Number of shares issued In exchange per share (in shares)	$ 207.0
Share price (USD per share)	$ 1,887.16
Consideration paid	$ 264,500,000
Consideration received, common stock	217,200,000
AIM Bancshares, Inc. | Holders Of In-The-Money Options
Business Acquisition [Line Items]
Consideration received, common stock	$ 4,900,000