Acquisitions - Narrative (Details) $ / shares in Units, $ in Thousands			1 Months Ended	12 Months Ended
	May 30, 2017 USD ($) $ / shares shares	Aug. 12, 2016 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Nov. 17, 2016 office	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Cash received by Cascade shareholders (in dollars per share) | $ / shares	$ 1.91
Common stock issued (in shares) | shares	11,300,000
Price per share of common stock issued (in dollars per share) | $ / shares	$ 34.3
Cash paid related to Cascade options and restricted stock	$ 9,300
Cash paid for Cascade unvested restricted stock	$ 2,200
Shares issued for cascade unvested restricted stock | shares	168,000
Expense related to restricted awards excluded from consideration paid	$ 2,400
Restricted stock expense included In consideration paid	$ 5,500
Goodwill				$ 444,704	$ 212,820	$ 204,500		$ 205,600
Number of operating segments | segment				1
Estimated useful lives of related deposits	10 years
Acquisition related expenses				$ 27,156	$ 2,821	795
Legal and professional fees directly related to merger				6,500
Adjustment to pro forma net income for other costs				21,800
Cascade Bank
Business Acquisition [Line Items]
Number of banking offices | office							46
Core Deposits Intangibles
Business Acquisition [Line Items]
Finite-lived intangible assets acquired	$ 48,000	$ 2,500				$ 700
Cascade Bank
Business Acquisition [Line Items]
Acquisition related expenses				27,200
Legal and professional fees				$ 9,600
Revenue reported by Cascade			$ 12,900
Net income reported by Cascade			$ 3,000
Cascade Bank | Class A Common Stock
Business Acquisition [Line Items]
Common stock received by Cascade shareholders (in shares) | shares	0.14864
Common stock issued (in shares) | shares	11,252,750
Cascade Bank Acquisition, as Recorded by the Company
Business Acquisition [Line Items]
Cash paid as consideration	$ 155,000
Total consideration paid	541,000
Goodwill	231,900
Cascade Bank Acquisition, as Recorded by the Company | Core Deposits Intangibles
Business Acquisition [Line Items]
Finite-lived intangible assets acquired	$ 48,000
Flathead Bank
Business Acquisition [Line Items]
Cash paid as consideration		34,100
Flathead Bank | Core Deposits Intangibles
Business Acquisition [Line Items]
Finite-lived intangible assets acquired		$ 2,500
Estimated useful lives of related deposits		9 years