Subordinated Debentures Held by Subsidiary Trusts (Details) $ in Thousands	1 Months Ended				12 Months Ended
	Jan. 31, 2008 USD ($)	Dec. 31, 2007 USD ($)	Nov. 30, 2007 USD ($)	Oct. 31, 2007 USD ($)	Dec. 31, 2017 USD ($) trusts	Dec. 31, 2016 USD ($)
Subordinated Borrowing
Number of company sponsored wholly-owned business trusts | trusts					6
Subordinated debentures held by subsidiary trusts					$ 82,477	$ 82,477
First Interstate Statutory Trust II | Junior Subordinated Deferrable Interest Debentures Issued by FIST II
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts				$ 10,300	$ 10,300	10,300
Debt instrument, interest rate at period end					3.94%
First Interstate Statutory Trust II | Junior Subordinated Deferrable Interest Debentures Issued by FIST II | LIBOR
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			$ 15,500		$ 15,500	15,500
Debt instrument, interest rate at period end					4.34%
First Interstate Statutory Trust I | Junior Subordinated Deferrable Interest Debentures Issued by FIST I | Debt Instrument, Redemption, At Issuance through Year Five
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		$ 20,600			$ 20,600	20,600
Debt instrument, interest rate at period end					3.99%
First Interstate Statutory Trust III | Junior Subordinated Deferrable Interest Debentures Issued by FIST III | Debt Instrument, Redemption, At Issuance through Year Five
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		$ 15,500			$ 15,500	15,500
Debt instrument, interest rate at period end					4.39%
First Interstate Statutory Trust IV | Junior Subordinated Deferrable Interest Debentures Issued by FIST IV | LIBOR
Subordinated Borrowing
Debt instrument, basis spread on variable rate		2.70%
First Interstate Statutory Trust V | Junior Subordinated Deferrable Interest Debentures Issued by FIST V
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts	$ 10,300				$ 10,300	10,300
Debt instrument, interest rate at period end					4.44%
First Interstate Statutory Trust V | Junior Subordinated Deferrable Interest Debentures Issued by FIST V | Debt Instrument, Redemption, At Issuance through Year Five
Subordinated Borrowing
Stated interest rate	6.78%
First Interstate Statutory Trust V | Junior Subordinated Deferrable Interest Debentures Issued by FIST V | LIBOR | Debt Instrument, Redemption, After Period Five
Subordinated Borrowing
Debt instrument, basis spread on variable rate	2.75%
First Interstate Statutory Trust VI | Junior Subordinated Deferrable Interest Debentures Issued by FIST VI
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts	$ 10,300				$ 10,300	$ 10,300
Debt instrument, interest rate at period end					4.44%
First Interstate Statutory Trust VI | Junior Subordinated Deferrable Interest Debentures Issued by FIST VI | LIBOR
Subordinated Borrowing
Debt instrument, basis spread on variable rate	2.75%
Trust Preferred Securities Subject to Mandatory Redemption | Trusts
Subordinated Borrowing
Amount of financial instruments subject to mandatory redemption					$ 80,000
Business trust term (in years)					30 years
Common Stock Subject to Mandatory Redemption | Trusts
Subordinated Borrowing
Amount of financial instruments subject to mandatory redemption					$ 2,500
Subordinated Debentures Subject to Mandatory Redemption | Trusts
Subordinated Borrowing
Business trust term (in years)					30 years