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GOODWILL, CORE DEPOSIT PREMIUM AND OTHER INTANGIBLE ASSETS (Goodwill narrative) (Details) - USD ($)		3 Months Ended
	Sep. 30, 2018	Mar. 31, 2019	Dec. 31, 2018	May 18, 2018	Feb. 23, 2018
Goodwill [Line Items]
Goodwill		$ 391,668,000	$ 391,668,000
Goodwill impairment	$ 0
Core Deposits
Goodwill [Line Items]
Intangibles, amortization period		10 years
First Bank Lubbock Bancshares, Inc.
Goodwill [Line Items]
Goodwill				$ 121,400,000
First Bank Lubbock Bancshares, Inc. \| Core Deposits
Goodwill [Line Items]
Intangibles recognized				$ 13,900,000
Signature Bancshares, Inc.
Goodwill [Line Items]
Goodwill					$ 33,700,000
Signature Bancshares, Inc. \| Core Deposits
Goodwill [Line Items]
Intangibles recognized					$ 7,700,000