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, 2018 USD ($) trusts	Dec. 31, 2017 USD ($)	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			$ 86.9	$ 82.5
Debt instrument, interest rate at period end			5.55%
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			5.05%
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			5.54%
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			5.19%
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			5.50%
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			$ 10.3	10.3	$ 15.5
Debt instrument, basis spread on variable rate			2.75%
Debt instrument, interest rate at period end			1030000000.00%
Northwest Bancorporation Capital Trust 1 [Member] | Junior Subordinated Deferrable Interest Debentures Issued by FIST VI
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts			$ 4.4	$ 0.0
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	4.50%
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			$ 84.3
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			$ 2.6
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