Goodwill and Intangible Assets Acquisitions (Details) - USD ($) $ in Thousands, shares in Millions				12 Months Ended
	Oct. 01, 2018	May 01, 2018	Dec. 01, 2016	Dec. 31, 2018	Dec. 31, 2016	Dec. 31, 2017
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Less Noncontrolling Interest [Abstract]
Goodwill				$ 1,049,263	$ 448,899	$ 447,430
CoBiz Financial Inc. [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition	Oct. 01, 2018
Name of Acquired Entity				CoBiz Financial, Inc. ("CoBiz")
Consideration Transferred				$ 944,193
Cash payments to acquire businesses				$ 243,000
Stock Issued During Period, Shares, Acquisitions				7.2
Equity interests issued				$ 701,000
Goodwill Recognized, Description				synergies expected to be gained through consolidation of administrative functions resulting in cost savings.
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Less Noncontrolling Interest [Abstract]
Cash and due from banks	$ 80,827
Investment securities	17,287
Available for sale securities	546,776
Restricted equity securities	5,261
Loans	2,937,499
Unpaid principal balance of loans	3,066,521
Premises and equipment	5,515
Receivables	24,893
Intangible assets	106,733
Real estate and other repossessed assets	5,155
Derivative contracts asset, net	8,197
Cash surrender value of bank-owned life insurance	55,740
Other assets	56,642
Total assets acquired	3,850,525
Deposits	3,289,071
Funds purchased and repurchase agreements	37,218
Subordinated debentures	131,197
Accrued interest, taxes and expense	33,122
Derivative contracts liability, Net	12,303
Other liabilities	5,254
Total liabilities assumed	3,508,165
Net assets acquired	342,360
Goodwill	$ 601,833
Switchgrass Holdings LLC [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition		May 01, 2018
Name of Acquired Entity				Switchgrass Holdings, LLC
Cash payments to acquire businesses				$ 14,000
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Less Noncontrolling Interest [Abstract]
Intangible assets				$ 6,700
MBT Bancshares [Member]
Business Acquisition [Line Items]
Effective Date of Acquisition			Dec. 01, 2016
Name of Acquired Entity					MBT Bancshares (“MBT”)
Cash payments to acquire businesses					$ 103,000
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Less Noncontrolling Interest [Abstract]
Intangible assets						15,000
Goodwill						$ 66,000