ACQUISITIONS (Details) $ / shares in Units, $ in Thousands	May 10, 2019 USD ($) $ / shares shares	May 18, 2018 USD ($) shares	Feb. 23, 2018 USD ($) shares	Sep. 30, 2019 USD ($) numberOfBankLocations	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Assets acquired				$ 12,569,262	$ 11,408,006
Gross loans held to maturity				7,971,608	7,407,697
Deposits acquired				10,469,856	$ 9,396,429
Blue Valley Ban Corp.
Business Acquisition [Line Items]
Share price (USD per share) | $ / shares	$ 44.78
Consideration paid	$ 92,300
Purchase price paid by delivery of shares of common stock (in shares) | shares	2,060,258
Outstanding debt repaid	$ 6,900
First Bank Lubbock Bancshares, Inc.
Business Acquisition [Line Items]
Assets acquired		$ 1,120,000
Consideration paid		$ 189,900
Purchase price paid by delivery of shares of common stock (in shares) | shares		3,350,664
Outstanding debt repaid		$ 3,900
Purchase price in cash		5,500
Signature Bancshares, Inc.
Business Acquisition [Line Items]
Consideration paid			$ 61,400
Purchase price paid by delivery of shares of common stock (in shares) | shares			1,000,843
Purchase price in cash			$ 7,800
Rockford Bank And Trust Company
Business Acquisition [Line Items]
Assets acquired				519,500
Gross loans held to maturity				417,300
Deposits acquired				$ 451,500
Number of bank locations | numberOfBankLocations				2
Blue Valley Ban Corp.
Business Acquisition [Line Items]
Assets acquired	766,200
Gross loans held to maturity	542,000
Deposits acquired	617,100
First Bank Lubbock Bancshares, Inc.
Business Acquisition [Line Items]
Gross loans held to maturity		681,100
Deposits acquired		893,800
Signature Bancshares, Inc.
Business Acquisition [Line Items]
Assets acquired			427,100
Gross loans held to maturity			324,500
Deposits acquired			357,300
Outstanding debt repaid			$ 5,900
Stock Appreciation Rights (SARs) | First Bank Lubbock Bancshares, Inc.
Business Acquisition [Line Items]
Payment to holders of FBLB's stock appreciation rights		13,300
Trust Preferred Securities Subject to Mandatory Redemption | Blue Valley Ban Corp.
Business Acquisition [Line Items]
Trust preferred securities assumed	$ 16,100
Trust Preferred Securities Subject to Mandatory Redemption | First Bank Lubbock Bancshares, Inc.
Business Acquisition [Line Items]
Trust preferred securities assumed		$ 8,200