Commitments and Contingent Liabilities (Details) - USD ($) $ in Thousands			12 Months Ended
	Aug. 11, 2017	Aug. 24, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Minimum future rental payments:
2016			$ 10,994
2017			10,458
2018			8,730
2019			7,104
2020			6,303
Thereafter			14,482
Total minimum payments			58,071
Operating Leases, Rent Expense, Sublease Rentals			791	$ 992	$ 1,100
Operating Leases, Rent Expense, Net			7,900	8,700	$ 7,400
Commitments to Extend Credit [Member]
Minimum future rental payments:
Fair Value Disclosure, Off-balance Sheet Risks, Amount, Liability			2,620,000	2,170,000
Standby Letters of Credit [Member]
Minimum future rental payments:
Fair Value Disclosure, Off-balance Sheet Risks, Amount, Liability			51,300	49,700
Commercial Letter of Credit and other off-balance sheet liabilities [Member]
Minimum future rental payments:
Fair Value Disclosure, Off-balance Sheet Risks, Amount, Liability			159	$ 3,400
IDAHO
Minimum future rental payments:
Sale Leaseback Transaction, Date	August 11, 2017
Sale Leaseback Transaction, Monthly Rental Payments			26
Sale Leaseback Transaction, Deferred Gain, Gross	$ 509
Sale Leaseback Transaction, Deferred Gain, Net			490
WASHINGTON
Minimum future rental payments:
Sale Leaseback Transaction, Date		August 24, 2016
Sale Leaseback Transaction, Monthly Rental Payments			12
Sale Leaseback Transaction, Deferred Gain, Gross		$ 742
Sale Leaseback Transaction, Deferred Gain, Net			$ 644