ACQUISITIONS (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 04, 2020	Jun. 30, 2021	Dec. 31, 2020
Business Acquisition [Line Items]
Assets acquired		$ 18,371,006	$ 17,908,339
Gross loans held to maturity		10,012,014	10,023,051
Deposits acquired		$ 15,615,118	$ 14,979,905
Share price (USD per share)	$ 41.89
Johnson Bank Branches | Arizona Bank & Trust
Business Acquisition [Line Items]
Assets acquired	$ 419,700
Gross loans held to maturity	150,700
Deposits acquired	415,500
AIM Bancshares, Inc.
Business Acquisition [Line Items]
Assets acquired	1,970,000
Gross loans held to maturity	1,090,000
Deposits acquired	$ 1,670,000
Number of shares issued In exchange per share (in shares)	$ 207.0
Share price (USD per share)	$ 1,887.16
Business combination, consideration transferred	$ 264,500
Consideration received, common stock	217,200
Payments to acquire businesses, gross	47,300
AIM Bancshares, Inc. | Holders Of In-The-Money Options
Business Acquisition [Line Items]
Consideration received, common stock	$ 4,900