Business Combination (Details) $ / shares in Units, shares in Thousands			1 Months Ended	3 Months Ended		6 Months Ended
	May 30, 2017 USD ($) $ / shares	Nov. 17, 2016	Jun. 30, 2017 USD ($) shares	Jun. 30, 2017 USD ($) $ / shares shares	Jun. 30, 2016 USD ($) $ / shares	Jun. 30, 2017 USD ($) $ / shares shares	Jun. 30, 2016 USD ($) $ / shares	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Cash paid related to Cascade Options						$ 9,300,000.0
Cash paid for Cascade unvested restricted stock			$ 2,200,000	$ 2,200,000		$ 2,200,000
Shares issued for Cascade unvested restricted stock | shares			168	168		168
Finite-lived Intangible Assets Acquired				$ 47,950,000	$ 0	$ 47,950,000	$ 0
Business Acquisition, Pro Forma Interest Income				103,248,000	94,698,000	203,326,000	189,772,000
Loans Acquired in Transfer, Contractually Required Payments Receivable at Acquisition			$ 2,098,155,000	2,098,155,000		2,098,155,000
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, Contractually Required Payments Receivable at Acquisition			48,041,000	48,041,000		48,041,000
Business Acquisition, Effective Date of Acquisition	May 30, 2017
Business Acquisition Conversion Rate	$ 1.91
Assets			12,236,342,000	12,236,342,000		12,236,342,000		$ 9,063,895,000
Deposits			10,020,000,000	10,020,000,000		10,020,000,000		7,376,110,000
Certain Loans Acquired in Transfer, Nonaccretable Difference			23,376,000	23,376,000		23,376,000
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, Cash Flows Expected to be Collected at Acquisition			24,665,000	24,665,000		24,665,000
Certain Loans Acquired in Transfer, Accretable Yield			1,901,000	1,901,000		1,901,000
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value			22,764,000	22,764,000		22,764,000
Loans Acquired in Transfer, Nonaccretable Difference			23,387,000	23,387,000		23,387,000
Loans Acquired in Transfer at Acquisition Fair Value			2,067,572,000	2,067,572,000		2,067,572,000
Business Acquisition, Pro Forma Non Interest Income				41,314,000	44,780,000	77,897,000	78,286,000
Business Acquisition, Pro Forma Revenue				144,562,000	139,478,000	281,223,000	268,058,000
Business Acquisition, Pro Forma Net Income (Loss)				$ 29,409,000	$ 29,534,000	$ 58,709,000	$ 23,716,000
Business Acquisition, Pro Forma Earnings Per Share, Basic | $ / shares				$ 0.50	$ 0.53	$ 1.02	$ 0.43
Business Acquisition, Pro Forma Earnings Per Share, Diluted | $ / shares				$ 500	$ 530	$ 1,010	$ 420
Non-core acquisition expenses				$ 10,133,000	$ 0	$ 10,838,000	$ 0
Professional Fees included in Acquisition Expense				5,926		6,542
Business Acquisition, Share Price | $ / shares	$ 34.30
Expense related to restricted awards excluded from consideration						2,400,000
Restricted stock expense included in consideration						5,500,000
Goodwill			$ 444,327,000	$ 444,327,000		$ 444,327,000		$ 212,820,000
Bank of the Cascades [Member]
Business Acquisition [Line Items]
Number of banking offices			46	46		46
Cascade Bank Acquisition, as Recorded by the Company [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross	$ 155,029,000
Business Combination, Consideration Transferred	540,998,000
Goodwill	$ 231,507,000
Flathead Bank [Member]
Business Acquisition [Line Items]
Business Acquisition, Date of Acquisition Agreement		Nov. 17, 2016
Bank of the Cascades [Member]
Business Acquisition [Line Items]
Business Acquisition, Revenue Reported by Acquired Entity for Reported Period			$ 12.9
Business Acquisition, Net Income Reported by Acquired Entity for Reported Period			3.0
Goodwill			$ 231,500	$ 231,500		$ 231,500
Bank of the Cascades [Member] | Class A Common Stock
Business Acquisition [Line Items]
Business Acquisition Conversion Share Rate | $ / shares	$ 0.14864
Bank of the Cascades [Member] | Core Deposits [Member]
Business Acquisition [Line Items]
Finite-lived Intangible Assets Acquired						$ 47,950,000
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life						10 years