Goodwill, Core Deposit Intangibles and Other Intangible Assets (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	May 18, 2018	Feb. 23, 2018	Dec. 31, 2017	Jul. 07, 2017	Feb. 28, 2017
Goodwill [Line Items]
Goodwill resulting from acquisition	$ 391,668,000			$ 236,615,000
Goodwill, impairment loss	$ 0
First Bank Lubbock Bancshares, Inc.
Goodwill [Line Items]
Goodwill resulting from acquisition		$ 121,363,000
First Bank Lubbock Bancshares, Inc. | Core Deposit Intangibles
Goodwill [Line Items]
Recognized intangibles		$ 13,900,000
Intangibles amortization period	10 years
Signature Bancshares, Inc.
Goodwill [Line Items]
Goodwill resulting from acquisition			$ 33,700,000
Intangibles amortization period	10 years
Signature Bancshares, Inc. | Core Deposit Intangibles
Goodwill [Line Items]
Recognized intangibles			$ 7,700,000
Citywide Banks of Colorado, Inc.
Goodwill [Line Items]
Goodwill resulting from acquisition					$ 95,154,000
Citywide Banks of Colorado, Inc. | Core Deposit Intangibles
Goodwill [Line Items]
Recognized intangibles					$ 16,000,000
Intangibles amortization period	10 years
Founders Bancorp
Goodwill [Line Items]
Goodwill resulting from acquisition						$ 13,800,000
Founders Bancorp | Core Deposit Intangibles
Goodwill [Line Items]
Recognized intangibles						$ 2,500,000
Intangibles amortization period	10 years