Subordinated Debentures Held by Subsidiary Trusts (Details) $ in Millions		1 Months Ended	12 Months Ended
	Aug. 16, 2018	Dec. 31, 2007	Dec. 31, 2022 USD ($) trusts	Dec. 31, 2011	Dec. 31, 2021 USD ($)	May 31, 2020	Nov. 30, 2007
Subordinated Borrowing
Number of company sponsored wholly-owned business trusts | trusts			14
Subordinated debentures held by subsidiary trusts					$ 87.0
Stated interest rate						5.25%
Subordinated Debt			$ 163.7		87.7
Subordinated Long-term Debt, Noncurrent			163.1		87.0
Common Stock Held in Assets			5.3		2.7
Subordinated Debt Fair Value Adjustment			(0.6)		(0.7)
Junior Subordinated Deferrable Interest Debentures Issued by GWB Trust II
Subordinated Borrowing
Subordinated Debt			2.0
Junior Subordinated Deferrable Interest Debentures Issued by INB Cap Trust I
Subordinated Borrowing
Subordinated Debt			5.0
Junior Subordinated Deferrable Interest Debentures Issued by GWB Trust VI
Subordinated Borrowing
Subordinated Debt			30.0
Junior Subordinated Deferrable Interest Debentures Issued by GWB Trust IV
Subordinated Borrowing
Subordinated Debt			22.4
Junior Subordinated Deferrable Interest Debentures Issued by HFFC III
Subordinated Borrowing
Subordinated Debt			5.0
Junior Subordinated Deferrable Interest Debentures Issued by HFFC IV
Subordinated Borrowing
Subordinated Debt			7.0
Junior Subordinated Deferrable Interest Debentures Issued by HFFC V
Subordinated Borrowing
Subordinated Debt			5.0
Junior Subordinated Deferrable Interest Debentures Issued by HFFC VI
Subordinated Borrowing
Subordinated Debt			2.0
First Interstate Statutory Trust II | Junior Subordinated Deferrable Interest Debentures Issued by FIST II
Subordinated Borrowing
Subordinated Debt					10.3
First Interstate Statutory Trust III | Junior Subordinated Deferrable Interest Debentures Issued by FIST III
Subordinated Borrowing
Subordinated Debt					20.6
First Interstate Statutory Trust IV | Junior Subordinated Deferrable Interest Debentures Issued by FIST IV
Subordinated Borrowing
Subordinated Debt					15.5
First Interstate Statutory Trust V | Junior Subordinated Deferrable Interest Debentures Issued by FIST V
Subordinated Borrowing
Subordinated Debt					10.3
First Interstate Statutory Trust VI | Junior Subordinated Deferrable Interest Debentures Issued by FIST VI
Subordinated Borrowing
Subordinated Debt					10.3
Northwest Bancorporation Capital Trust 1 [Member] | Junior Subordinated Deferrable Interest Debentures Issued by FIST VI
Subordinated Borrowing
Subordinated Debt					5.2
Junior Subordinated Deferrable Interest Debentures Issued by GWB Trust II
Subordinated Borrowing
Common Stock Held in Assets			0.1		0.0
Junior Subordinated Deferrable Interest Debentures Issued by GWB Trust II | Junior Subordinated Deferrable Interest Debentures Issued by GWB Trust II
Subordinated Borrowing
Subordinated Debt			$ 2.1		0.0
Debt instrument, basis spread on variable rate				1.85%
Junior Subordinated Deferrable Interest Debentures Issued by GWB Trust II | Junior Subordinated Deferrable Interest Debentures Issued by GWB Trust II | LIBOR
Subordinated Borrowing
Debt instrument, interest rate at period end			6.62%
Junior Subordinated Deferrable Interest Debentures Issued by FIST II
Subordinated Borrowing
Common Stock Held in Assets			$ 0.3		0.3
Junior Subordinated Deferrable Interest Debentures Issued by FIST II | Junior Subordinated Deferrable Interest Debentures Issued by FIST II
Subordinated Borrowing
Subordinated Debt			$ 10.3
Junior Subordinated Deferrable Interest Debentures Issued by FIST II | Junior Subordinated Deferrable Interest Debentures Issued by FIST II | Debt Instrument, Redemption, After Period Five
Subordinated Borrowing
Debt instrument, basis spread on variable rate		2.25%
Junior Subordinated Deferrable Interest Debentures Issued by FIST II | Junior Subordinated Deferrable Interest Debentures Issued by FIST II | LIBOR
Subordinated Borrowing
Debt instrument, interest rate at period end			5.99%
Junior Subordinated Deferrable Interest Debentures Issued by FIST I
Subordinated Borrowing
Common Stock Held in Assets			$ 0.5		0.5
Junior Subordinated Deferrable Interest Debentures Issued by FIST I | Junior Subordinated Deferrable Interest Debentures Issued by FIST I
Subordinated Borrowing
Subordinated Debt			$ 15.5		15.5
Junior Subordinated Deferrable Interest Debentures Issued by FIST I | Junior Subordinated Deferrable Interest Debentures Issued by FIST I | Debt Instrument, Redemption, After Period Five
Subordinated Borrowing
Debt instrument, basis spread on variable rate		2.75%
Junior Subordinated Deferrable Interest Debentures Issued by FIST 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%
Junior Subordinated Deferrable Interest Debentures Issued by FIST I | Junior Subordinated Deferrable Interest Debentures Issued by FIST I | LIBOR
Subordinated Borrowing
Debt instrument, interest rate at period end			7.52%
Junior Subordinated Deferrable Interest Debentures Issued by FIST III
Subordinated Borrowing
Common Stock Held in Assets			$ 0.6		0.6
Junior Subordinated Deferrable Interest Debentures Issued by FIST III | Junior Subordinated Deferrable Interest Debentures Issued by FIST III
Subordinated Borrowing
Subordinated Debt			$ 20.6
Junior Subordinated Deferrable Interest Debentures Issued by FIST III | Junior Subordinated Deferrable Interest Debentures Issued by FIST III | Debt Instrument, Redemption, After Period Five
Subordinated Borrowing
Debt instrument, basis spread on variable rate		2.40%
Junior Subordinated Deferrable Interest Debentures Issued by FIST 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%
Junior Subordinated Deferrable Interest Debentures Issued by FIST III | Junior Subordinated Deferrable Interest Debentures Issued by FIST III | LIBOR
Subordinated Borrowing
Debt instrument, interest rate at period end			7.17%
Junior Subordinated Deferrable Interest Debentures Issued by FIST IV
Subordinated Borrowing
Common Stock Held in Assets			$ 0.5		0.5
Junior Subordinated Deferrable Interest Debentures Issued by FIST IV | Junior Subordinated Deferrable Interest Debentures Issued by FIST IV
Subordinated Borrowing
Debt instrument, interest rate at period end		2.70%
Subordinated Debt			$ 15.5
Junior Subordinated Deferrable Interest Debentures Issued by FIST IV | Junior Subordinated Deferrable Interest Debentures Issued by FIST IV | LIBOR
Subordinated Borrowing
Debt instrument, interest rate at period end			6.44%
Junior Subordinated Deferrable Interest Debentures Issued by FIST V
Subordinated Borrowing
Common Stock Held in Assets			$ 0.3		0.3
Junior Subordinated Deferrable Interest Debentures Issued by FIST V | Junior Subordinated Deferrable Interest Debentures Issued by FIST V
Subordinated Borrowing
Subordinated Debt			$ 10.3
Junior Subordinated Deferrable Interest Debentures Issued by FIST V | Junior Subordinated Deferrable Interest Debentures Issued by FIST V | Debt Instrument, Redemption, After Period Five
Subordinated Borrowing
Debt instrument, basis spread on variable rate				2.75%
Junior Subordinated Deferrable Interest Debentures Issued by FIST V | Junior Subordinated Deferrable Interest Debentures Issued by FIST V | Debt Instrument, Redemption, At Issuance through Year Five [Member]
Subordinated Borrowing
Stated interest rate		6.78%
Junior Subordinated Deferrable Interest Debentures Issued by FIST V | Junior Subordinated Deferrable Interest Debentures Issued by FIST V | LIBOR
Subordinated Borrowing
Debt instrument, interest rate at period end			6.49%
Junior Subordinated Deferrable Interest Debentures Issued by FIST VI
Subordinated Borrowing
Common Stock Held in Assets			$ 0.3		0.3
Junior Subordinated Deferrable Interest Debentures Issued by FIST VI | Junior Subordinated Deferrable Interest Debentures Issued by FIST VI
Subordinated Borrowing
Subordinated Debt			$ 10.3
Debt instrument, basis spread on variable rate				2.75%
Junior Subordinated Deferrable Interest Debentures Issued by FIST VI | Junior Subordinated Deferrable Interest Debentures Issued by FIST VI | LIBOR
Subordinated Borrowing
Debt instrument, interest rate at period end			6.49%
Junior Subordinated Deferrable Interest Debentures Issued by INB Cap Trust I
Subordinated Borrowing
Common Stock Held in Assets			$ 0.2		0.2
Junior Subordinated Deferrable Interest Debentures Issued by INB Cap Trust I | Junior Subordinated Deferrable Interest Debentures Issued by INB Cap Trust I
Subordinated Borrowing
Subordinated Debt			$ 5.2
Junior Subordinated Deferrable Interest Debentures Issued by INB Cap Trust I | Junior Subordinated Deferrable Interest Debentures Issued by INB Cap Trust I | Debt Instrument, Redemption, After Period Five
Subordinated Borrowing
Debt instrument, basis spread on variable rate	1.70%
Junior Subordinated Deferrable Interest Debentures Issued by INB Cap Trust I | Junior Subordinated Deferrable Interest Debentures Issued by INB Cap Trust I | Debt Instrument, Redemption, At Issuance through Year Five [Member]
Subordinated Borrowing
Stated interest rate		5.95%
Junior Subordinated Deferrable Interest Debentures Issued by INB Cap Trust I | Junior Subordinated Deferrable Interest Debentures Issued by INB Cap Trust I | LIBOR
Subordinated Borrowing
Debt instrument, interest rate at period end			6.43%
Junior Subordinated Deferrable Interest Debentures Issued by GWB Trust VI
Subordinated Borrowing
Common Stock Held in Assets			$ 0.9		0.0
Junior Subordinated Deferrable Interest Debentures Issued by GWB Trust VI | Junior Subordinated Deferrable Interest Debentures Issued by GWB Trust VI
Subordinated Borrowing
Subordinated Debt			$ 30.9		0.0
Debt instrument, basis spread on variable rate				1.48%
Junior Subordinated Deferrable Interest Debentures Issued by GWB Trust VI | Junior Subordinated Deferrable Interest Debentures Issued by GWB Trust VI | LIBOR
Subordinated Borrowing
Debt instrument, interest rate at period end			6.25%
Junior Subordinated Deferrable Interest Debentures Issued by GWB Trust IV
Subordinated Borrowing
Common Stock Held in Assets			$ 0.7		0.0
Junior Subordinated Deferrable Interest Debentures Issued by GWB Trust IV | Junior Subordinated Deferrable Interest Debentures Issued by GWB Trust IV
Subordinated Borrowing
Subordinated Debt			$ 23.1		0.0
Debt instrument, basis spread on variable rate				2.85%
Junior Subordinated Deferrable Interest Debentures Issued by GWB Trust IV | Junior Subordinated Deferrable Interest Debentures Issued by GWB Trust IV | LIBOR
Subordinated Borrowing
Debt instrument, interest rate at period end			7.59%
Junior Subordinated Deferrable Interest Debentures Issued by HFFC III
Subordinated Borrowing
Common Stock Held in Assets			$ 0.2		0.0
Junior Subordinated Deferrable Interest Debentures Issued by HFFC III | Junior Subordinated Deferrable Interest Debentures Issued by HFFC III
Subordinated Borrowing
Subordinated Debt			$ 5.2		0.0
Debt instrument, basis spread on variable rate				3.35%
Junior Subordinated Deferrable Interest Debentures Issued by HFFC III | Junior Subordinated Deferrable Interest Debentures Issued by HFFC III | LIBOR
Subordinated Borrowing
Debt instrument, interest rate at period end			7.43%
Junior Subordinated Deferrable Interest Debentures Issued by HFFC IV
Subordinated Borrowing
Common Stock Held in Assets			$ 0.2		0.0
Junior Subordinated Deferrable Interest Debentures Issued by HFFC IV | Junior Subordinated Deferrable Interest Debentures Issued by HFFC IV
Subordinated Borrowing
Subordinated Debt			$ 7.2		0.0
Debt instrument, basis spread on variable rate				3.10%
Junior Subordinated Deferrable Interest Debentures Issued by HFFC IV | Junior Subordinated Deferrable Interest Debentures Issued by HFFC IV | LIBOR
Subordinated Borrowing
Debt instrument, interest rate at period end			7.18%
Junior Subordinated Deferrable Interest Debentures Issued by HFFC V
Subordinated Borrowing
Common Stock Held in Assets			$ 0.3		0.0
Junior Subordinated Deferrable Interest Debentures Issued by HFFC V | Junior Subordinated Deferrable Interest Debentures Issued by HFFC V
Subordinated Borrowing
Subordinated Debt			$ 5.3		0.0
Debt instrument, basis spread on variable rate				1.83%
Junior Subordinated Deferrable Interest Debentures Issued by HFFC V | Junior Subordinated Deferrable Interest Debentures Issued by HFFC V | LIBOR
Subordinated Borrowing
Debt instrument, interest rate at period end			6.59%
Junior Subordinated Deferrable Interest Debentures Issued by HFFC VI
Subordinated Borrowing
Common Stock Held in Assets			$ 0.2		0.0
Junior Subordinated Deferrable Interest Debentures Issued by HFFC VI | Junior Subordinated Deferrable Interest Debentures Issued by HFFC VI
Subordinated Borrowing
Subordinated Debt			$ 2.2		$ 0.0
Debt instrument, basis spread on variable rate				1.65%
Junior Subordinated Deferrable Interest Debentures Issued by HFFC VI | Junior Subordinated Deferrable Interest Debentures Issued by HFFC VI | LIBOR
Subordinated Borrowing
Debt instrument, interest rate at period end			5.39%
Trust Preferred Securities Subject to Mandatory Redemption | Trusts
Subordinated Borrowing
Business trust term (in years)			30 years
Common Stock Subject to Mandatory Redemption | Trusts
Subordinated Borrowing
Amount of financial instruments subject to mandatory redemption			$ 5.3
Subordinated Debentures Subject to Mandatory Redemption | Trusts
Subordinated Borrowing
Business trust term (in years)			30 years