Acquisitions (Narrative) (Details) $ in Thousands					3 Months Ended	6 Months Ended	12 Months Ended
	Feb. 23, 2018 USD ($) shares	Jul. 07, 2017 USD ($) building shares	Feb. 28, 2017 USD ($) shares	Feb. 05, 2016 USD ($) shares	Jun. 30, 2017 USD ($) building	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Assets							$ 9,810,739	$ 8,247,079	$ 7,694,754
Held to maturity							6,391,464	5,351,719
Deposits							8,146,909	6,847,411
Loans outstanding							6,335,778	5,297,395
Goodwill							236,615	127,699
Nonaccrual loans							58,272	62,591
Fair value of one bank building held for sale							1,977	414
Signature Bancshares, Inc.
Business Acquisition [Line Items]
Assets							409,200
Held to maturity							339,100
Deposits							368,100
First Bank Lubbock Bancshares, Inc.
Business Acquisition [Line Items]
Assets							929,600
Held to maturity							669,300
Deposits							821,900
Citywide Banks of Colorado, Inc.
Business Acquisition [Line Items]
Total consideration paid		$ 211,243
Cash payments to acquire business		58,636
Assets		1,490,000
Deposits		$ 1,210,000
Business acquisition, number of shares issued (in shares) | shares		3,216,161
Loans outstanding		$ 985,400
Number of buildings | building		1
Fair value of sold building		$ 1,400
Goodwill		95,154
Merger related expenses							3,800
Nonaccrual loans		1,200
Other borrowings		$ 21,636
Founders Bancorp
Business Acquisition [Line Items]
Total consideration paid			$ 31,000
Cash payments to acquire business			8,400
Assets			213,900
Deposits			$ 181,500
Business acquisition, number of shares issued (in shares) | shares			455,877
Number of buildings | building					1
Fair value of sold building					$ 576
Goodwill			$ 13,800
Estimated fair value of loans acquired			$ 96,400
CIC Bancshares, Inc.
Business Acquisition [Line Items]
Total consideration paid				$ 76,882
Cash payments to acquire business				15,672
Assets				772,600
Deposits				$ 648,100
Business acquisition, number of shares issued (in shares) | shares				2,003,235
Goodwill				$ 29,848
Merger related expenses								$ 551
Nonaccrual loans				1,600
Other borrowings				7,924
Estimated fair value of loans acquired				581,500
CIC Bancshares, Inc. | Preferred Stock
Business Acquisition [Line Items]
Common stock				$ 3,800
Transaction costs | Citywide Banks of Colorado, Inc.
Business Acquisition [Line Items]
Transaction costs							$ (10,100)
Common Stock | CIC Bancshares, Inc.
Business Acquisition [Line Items]
Business acquisition, number of shares issued (in shares) | shares				2,003,235
Subsequent Event | Signature Bancshares, Inc.
Business Acquisition [Line Items]
Total consideration paid	$ 61,400
Cash payments to acquire business	$ 7,700
Business acquisition, number of shares issued (in shares) | shares	1,001,246
Scenario, Forecast | First Bank Lubbock Bancshares, Inc.
Business Acquisition [Line Items]
Total consideration paid						$ 185,600