Subordinated Debentures Held by Subsidiary Trusts (Details) $ in Millions		1 Months Ended	12 Months Ended
	Aug. 16, 2018 USD ($)	Dec. 31, 2007 USD ($)	Dec. 31, 2021 USD ($) trusts	Dec. 31, 2010	Dec. 31, 2020 USD ($)	May 31, 2020	Jan. 31, 2008 USD ($)	Nov. 30, 2007	Oct. 31, 2007 USD ($)
Subordinated Borrowing
Number of company sponsored wholly-owned business trusts | trusts			7
Subordinated debentures held by subsidiary trusts			$ 87.0		$ 87.0
Debt instrument, interest rate at period end			2.88%
Stated interest rate						5.25%
Debt Instrument, Redemption, At Issuance through Year Five [Member]
Subordinated Borrowing
Stated interest rate							6.78%
LIBOR | Debt Instrument, Redemption, After Period Five
Subordinated Borrowing
Debt instrument, basis spread on variable rate				2.75%
First Interstate Statutory Trust II | Junior Subordinated Deferrable Interest Debentures Issued by FIST II
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts			$ 10.3		10.3				$ 10.3
Debt instrument, interest rate at period end			2.38%
First Interstate Statutory Trust II | Junior Subordinated Deferrable Interest Debentures Issued by FIST II | LIBOR | Debt Instrument, Redemption, After Period Five
Subordinated Borrowing
Debt instrument, basis spread on variable rate		2.25%
First Interstate Statutory Trust I | Junior Subordinated Deferrable Interest Debentures Issued by FIST I
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts			$ 15.5		15.5				$ 15.5
Debt instrument, interest rate at period end			2.95%
First Interstate Statutory Trust I | Junior Subordinated Deferrable Interest Debentures Issued by FIST I | Debt Instrument, Redemption, At Issuance through Year Five [Member]
Subordinated Borrowing
Stated interest rate								7.50%
First Interstate Statutory Trust I | Junior Subordinated Deferrable Interest Debentures Issued by FIST I | LIBOR | Debt Instrument, Redemption, After Period Five
Subordinated Borrowing
Debt instrument, basis spread on variable rate		2.75%
First Interstate Statutory Trust III | Junior Subordinated Deferrable Interest Debentures Issued by FIST III
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts		$ 20.6	$ 20.6		20.6
Debt instrument, interest rate at period end			2.60%
First Interstate Statutory Trust III | Junior Subordinated Deferrable Interest Debentures Issued by FIST III | Debt Instrument, Redemption, At Issuance through Year Five [Member]
Subordinated Borrowing
Stated interest rate		6.88%
First Interstate Statutory Trust III | Junior Subordinated Deferrable Interest Debentures Issued by FIST III | LIBOR | Debt Instrument, Redemption, After Period Five
Subordinated Borrowing
Debt instrument, basis spread on variable rate		2.40%
First Interstate Statutory Trust IV | Junior Subordinated Deferrable Interest Debentures Issued by FIST IV
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts			$ 15.5		15.5
First Interstate Statutory Trust V | Junior Subordinated Deferrable Interest Debentures Issued by FIST V
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts			$ 10.3		10.3		$ 10.3
First Interstate Statutory Trust VI | Junior Subordinated Deferrable Interest Debentures Issued by FIST IV
Subordinated Borrowing
Debt instrument, basis spread on variable rate			2.83%
Debt instrument, interest rate at period end		2.70%
First Interstate Statutory Trust VI | Junior Subordinated Deferrable Interest Debentures Issued by FIST VI
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts		$ 15.5	$ 10.3		10.3		$ 10.3
Debt instrument, basis spread on variable rate			2.88%	2.75%
Northwest Bancorporation Capital Trust 1 [Member] | Junior Subordinated Deferrable Interest Debentures Issued by FIST VI
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts			$ 4.5		$ 4.5
Northwest Bancorporation Capital Trust I [Member] | Junior Subordinated Deferrable Interest Debentures Issued by NWBC I [Member]
Subordinated Borrowing
Debt instrument, basis spread on variable rate	1.70%
Debt instrument, interest rate at period end			1.92%
Northwest Bancorporation Capital Trust I [Member] | Junior Subordinated Deferrable Interest Debentures Issued by NWBC I [Member] | Debt Instrument, Redemption, At Issuance through Year Five [Member]
Subordinated Borrowing
Stated interest rate	5.95%
Trust Preferred Securities Subject to Mandatory Redemption | Trusts
Subordinated Borrowing
Amount of financial instruments subject to mandatory redemption			$ 84.2
Business trust term (in years)			30 years
Trust Preferred Securities Subject to Mandatory Redemption | Northwest Bancorporation Capital Trust 1 [Member]
Subordinated Borrowing
Amount of financial instruments subject to mandatory redemption	$ 5.0
Common Stock Subject to Mandatory Redemption | Trusts
Subordinated Borrowing
Amount of financial instruments subject to mandatory redemption			$ 2.8
Common Stock Subject to Mandatory Redemption | Northwest Bancorporation Capital Trust I [Member]
Subordinated Borrowing
Amount of financial instruments subject to mandatory redemption	$ 0.2
Subordinated Debentures Subject to Mandatory Redemption | Trusts
Subordinated Borrowing
Business trust term (in years)			30 years