Subordinated Debentures Held by Subsidiary Trusts - Subordinated Debentures (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended	1 Months Ended				0 Months Ended
	Dec. 31, 2014 trusts	Oct. 31, 2007	Nov. 30, 2007	Dec. 31, 2007	Jan. 31, 2008	Dec. 26, 2014	Dec. 15, 2014	Dec. 31, 2013
Subordinated Borrowing
Number of company sponsored wholly-owned business 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	10,310	10,310						10,310
Debt instrument, interest rate at period end	2.49%
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	15,464		15,464					15,464
Debt instrument, interest rate at period end	2.99%
First Interstate Statutory Trust I | Junior Subordinated Deferrable Interest Debentures Issued by FIST I | Debt Instrument, Redemption, At Issuance through Year Five
Subordinated Borrowing
Debt instrument, interest rate, stated percentage			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	20,619			20,619				20,619
Debt instrument, interest rate at period end	2.64%
First Interstate Statutory Trust III | Junior Subordinated Deferrable Interest Debentures Issued by FIST III | Debt Instrument, Redemption, At Issuance through Year Five
Subordinated Borrowing
Debt instrument, interest rate, stated percentage				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	15,464							15,464
Debt instrument, interest rate at period end	2.94%
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	10,310				10,310			10,310
Debt instrument, interest rate at period end	2.99%
First Interstate Statutory Trust V | Junior Subordinated Deferrable Interest Debentures Issued by FIST V | Debt Instrument, Redemption, At Issuance through Year Five
Subordinated Borrowing
Debt instrument, interest rate, stated percentage					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	10,310				10,310			10,310
Debt instrument, interest rate at period end	2.99%
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%
Mountain West Trusts [Member] | Junior Subordinated Deferrable Interest Debenture Issues by MWFC [Member]
Subordinated Borrowing
Redemption price						6,006	14,433
Liquidation amount per security						1	1
Mandatory redemption of trust preferred securities						5,825	14,000
Capitalization of subsidiaries						181	433
Mountain West Trusts [Member] | Junior Subordinated Deferrable Interest Debenture Issues by MWFC [Member] | LIBOR
Subordinated Borrowing
Debt instrument, basis spread on variable rate						3.10%	1.85%
Trust Preferred Securities Subject to Mandatory Redemption | Mountain West Trusts [Member]
Subordinated Borrowing
Financial Instruments Subject to Mandatory Redemption, Settlement Terms, Share Value, Amount	19,825
Common Stock Subject to Mandatory Redemption | Mountain West Trusts [Member]
Subordinated Borrowing
Financial Instruments Subject to Mandatory Redemption, Settlement Terms, Share Value, Amount	$ 614