Acquisitions (Narrative) (Details) $ in Thousands			3 Months Ended
	Feb. 28, 2017 USD ($) building shares	Feb. 05, 2016 USD ($) shares	Sep. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)
Business Acquisition [Line Items]
Assets				$ 8,361,845	$ 8,247,079	$ 8,253,779
Loans outstanding				5,306,605	5,297,395
Deposits				7,089,861	6,847,411
Premises, furniture and equipment held for sale				1,242	$ 414
Founders Bancorp
Business Acquisition [Line Items]
Total purchase price	$ 31,000
Assets	213,900
Deposits	$ 181,500
Purchase price paid by delivery of shares of common stock (shares) | shares	455,877
Purchase price in cash	$ 8,400
Loans	$ 96,400
Number of bank buildings held-for-sale (building) | building	1
Premises, furniture and equipment held for sale	$ 576
CIC Bancshares, Inc.
Business Acquisition [Line Items]
Total purchase price		$ 76,900
Assets		772,600
Deposits		$ 648,100
Purchase price paid by delivery of shares of common stock (shares) | shares		2,003,235
Purchase price in cash		$ 15,700
Loans		581,500
Other borrowings		7,900
CIC Bancshares, Inc. | Preferred Stock
Business Acquisition [Line Items]
Convertible preferred stock issue, fair value		$ 3,800
Citywide Banks of Colorado, Inc. | Citywide Banks of Colorado, Inc.
Business Acquisition [Line Items]
Assets				1,350,000
Loans outstanding				982,000
Deposits				$ 1,170,000
Scenario, Forecast | Citywide Banks of Colorado, Inc.
Business Acquisition [Line Items]
Total purchase price			$ 203,000