Goodwill, Core Deposit Intangibles and Other Intangible Assets (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Nov. 30, 2019	May 10, 2019	Dec. 31, 2018	May 18, 2018	Feb. 23, 2018
Goodwill [Line Items]
Goodwill resulting from acquisition	$ 446,345,000			$ 391,668,000
Goodwill, impairment loss	$ 0
Rockford Bank and Trust Company
Goodwill [Line Items]
Goodwill resulting from acquisition		$ 19,200,000
Rockford Bank and Trust Company | Core Deposit Intangibles
Goodwill [Line Items]
Recognized intangibles		$ 1,800,000
Intangibles amortization period	10 years
Blue Valley Ban Corp.
Goodwill [Line Items]
Goodwill resulting from acquisition			$ 35,400,000
Blue Valley Ban Corp. | Core Deposit Intangibles
Goodwill [Line Items]
Recognized intangibles			$ 11,400,000
Intangibles amortization period	10 years
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
Signature Bancshares, Inc. | Core Deposit Intangibles
Goodwill [Line Items]
Recognized intangibles						$ 7,700,000
Intangibles amortization period	10 years