Acquisitions (Narrative) (Details) $ in Thousands						3 Months Ended	12 Months Ended
	Jan. 16, 2019 USD ($)	May 18, 2018 USD ($) shares	Feb. 23, 2018 USD ($) shares	Jul. 07, 2017 USD ($) shares	Feb. 28, 2017 USD ($) shares	Jun. 30, 2017 USD ($) building	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Assets							$ 11,408,006	$ 9,810,739
Total net loans receivable held to maturity							7,407,697	6,391,464
Deposits							9,396,429	8,146,909
Goodwill							391,668	236,615
Balance of nonaccrual loans							71,943	62,581
Loans outstanding							7,345,734	6,335,778
Blue Valley Ban Corp.
Business Acquisition [Line Items]
Assets							715,100
Total net loans receivable held to maturity							561,300
Deposits							562,600
Signature Bancshares, Inc.
Business Acquisition [Line Items]
Outstanding debt extinguished			$ 5,900
First Bank Lubbock Bancshares, Inc.
Business Acquisition [Line Items]
Total net loans receivable held to maturity		$ 681,100
Deposits		893,800
Signature Bancshares, Inc.
Business Acquisition [Line Items]
Total consideration paid			61,400
Cash payments to acquire business			$ 7,800
Business acquisition, number of shares issued (in shares) | shares			1,000,843
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]
Value of common stock issued in business acquisition		184,454
Total consideration paid		189,905
Cash payments to acquire business		$ 5,451
Business acquisition, number of shares issued (in shares) | shares		3,350,664
Assets acquired		$ 996,026
Loans held-to-maturity acquired		681,080
Deposits, assumed		893,827
Outstanding debt extinguished		3,900
Other borrowings		12,077
Total assets acquired		1,120,000
Goodwill		121,363
Transaction costs		6,000
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]
Other borrowings		$ 8,200
Citywide Banks of Colorado, Inc.
Business Acquisition [Line Items]
Value of common stock issued in business acquisition				$ 152,607
Assets				1,490,000
Deposits				1,210,000
Total consideration paid				211,243
Cash payments to acquire business				$ 58,636
Business acquisition, number of shares issued (in shares) | shares				3,216,161
Assets acquired				$ 1,398,539
Loans held-to-maturity acquired				985,399
Deposits, assumed				1,210,074
Other borrowings				21,636
Goodwill				95,154
Merger related expenses								$ 3,800
Balance of nonaccrual loans				1,200
Loans outstanding				$ 985,400
Founders Bancorp
Business Acquisition [Line Items]
Assets					$ 213,900
Deposits					181,500
Total consideration paid					31,000
Cash payments to acquire business					$ 8,400
Business acquisition, number of shares issued (in shares) | shares					455,877
Goodwill					$ 13,800
Number of buildings | building						1
Fair value of sold building						$ 576
Estimated fair value of loans acquired					$ 96,400
Subsequent Event | Blue Valley Ban Corp.
Business Acquisition [Line Items]
Value of common stock issued in business acquisition	$ 93,900