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Goodwill, Core Deposit Premium and Other Intangible Assets (Goodwill narrative) (Details) - USD ($)		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2018	May 18, 2018	Feb. 23, 2018	Dec. 31, 2017	Jul. 07, 2017	Feb. 28, 2017
Goodwill [Line Items]
Goodwill		$ 391,668,000			$ 236,615,000
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
Citywide Banks
Goodwill [Line Items]
Goodwill						$ 95,200,000
Citywide Banks \| Core Deposits
Goodwill [Line Items]
Intangibles recognized						$ 16,000,000
Founders Bancorp
Goodwill [Line Items]
Goodwill							$ 13,800,000
Founders Bancorp \| Core Deposits
Goodwill [Line Items]
Intangibles recognized							$ 2,500,000