Acquisitions (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands						12 Months Ended
	Feb. 11, 2020	Nov. 30, 2019	May 10, 2019	May 18, 2018	Feb. 23, 2018	Dec. 31, 2018	Dec. 31, 2019
Business Acquisition [Line Items]
Assets						$ 11,408,006	$ 13,209,597
Held to maturity						7,407,697	8,367,917
Deposits						9,396,429	11,044,331
Goodwill						391,668	446,345
Balance of nonaccrual loans						71,943	76,548
AIM Bancshares, Inc.
Business Acquisition [Line Items]
Assets							1,780,000
Held to maturity							1,160,000
Deposits							$ 1,540,000
Rockford Bank and Trust Company
Business Acquisition [Line Items]
Assets		$ 495,700
Held to maturity		354,000
Deposits		430,300
Cash payments to acquire business		46,600
Blue Valley Ban Corp.
Business Acquisition [Line Items]
Assets			$ 766,200
Held to maturity			542,000
Deposits			617,100
Signature Bancshares, Inc.
Business Acquisition [Line Items]
Outstanding debt extinguished					$ 5,900
First Bank Lubbock Bancshares, Inc.
Business Acquisition [Line Items]
Held to maturity				$ 681,100
Deposits				893,800
AIM Bancshares, Inc. | Subsequent Event
Business Acquisition [Line Items]
Total consideration paid	$ 280,400
Rockford Bank and Trust Company
Business Acquisition [Line Items]
Goodwill		$ 19,200
Blue Valley Ban Corp.
Business Acquisition [Line Items]
Total consideration paid			$ 92,300
Common stock price (in dollars per share)			$ 44.78
Business acquisition, number of shares issued (in shares)			2,060,258
Outstanding debt extinguished			$ 6,900
Goodwill			35,400
Blue Valley Ban Corp. | Trust preferred securities
Business Acquisition [Line Items]
Trust preferred securities			$ 16,100
Signature Bancshares, Inc.
Business Acquisition [Line Items]
Total consideration paid					$ 61,400
Business acquisition, number of shares issued (in shares)					1,000,843
Cash payments to acquire business					$ 7,800
Assets acquired					427,100
Loans held-to-maturity acquired					324,500
Deposits, assumed					357,300
Goodwill					$ 33,700
First Bank Lubbock Bancshares, Inc.
Business Acquisition [Line Items]
Total consideration paid				$ 189,905
Business acquisition, number of shares issued (in shares)				3,350,664
Outstanding debt extinguished				$ 3,900
Trust preferred securities				12,077
Cash payments to acquire business				5,451
Assets acquired				996,026
Loans held-to-maturity acquired				681,080
Deposits, assumed				893,827
Total assets acquired				1,120,000
Goodwill				121,363
Merger related expenses						1,100
Balance of nonaccrual loans						$ 7,600
First Bank Lubbock Bancshares, Inc. | Stock Appreciation Rights (SARs)
Business Acquisition [Line Items]
Stock appreciation rights expense				13,300
First Bank Lubbock Bancshares, Inc. | Trust preferred securities
Business Acquisition [Line Items]
Trust preferred securities				$ 8,200