ACQUISITIONS (Details) - USD ($) $ in Thousands	Jan. 16, 2019	May 18, 2018	Feb. 23, 2018	Mar. 31, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Assets acquired				$ 11,312,495	$ 11,408,006
Gross loans held to maturity				7,331,544	7,407,697
Deposits acquired				9,352,942	$ 9,396,429
Blue Valley Ban Corp.
Business Acquisition [Line Items]
Common stock exchanged on closing of the transaction	$ 93,900
First Bank Lubbock Bancshares, Inc.
Business Acquisition [Line Items]
Transaction value		$ 189,900
Purchase price in cash		$ 5,500
Purchase price paid by delivery of shares of common stock (in shares)		3,350,664
Outstanding debt repaid		$ 3,900
Total assets acquired		1,120,000
Signature Bancshares, Inc.
Business Acquisition [Line Items]
Transaction value			$ 61,400
Purchase price in cash			$ 7,800
Purchase price paid by delivery of shares of common stock (in shares)			1,000,843
Blue Valley Ban Corp.
Business Acquisition [Line Items]
Assets acquired				711,600
Gross loans held to maturity				564,100
Deposits acquired				$ 587,200
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 | First Bank Lubbock Bancshares, Inc.
Business Acquisition [Line Items]
Trust preferred securities assumed		$ 8,200