Subordinated Debentures Held by Subsidiary Trusts - Subordinated Debentures (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended					1 Months Ended					1 Months Ended					1 Months Ended			1 Months Ended					1 Months Ended				1 Months Ended	0 Months Ended
	Dec. 31, 2013 trusts	Dec. 31, 2012	Dec. 31, 2013 First Interstate Statutory Trust II Junior Subordinated Deferrable Interest Debentures Issued by FIST II	Dec. 31, 2012 First Interstate Statutory Trust II Junior Subordinated Deferrable Interest Debentures Issued by FIST II	Oct. 31, 2007 First Interstate Statutory Trust II Junior Subordinated Deferrable Interest Debentures Issued by FIST II	Oct. 31, 2007 First Interstate Statutory Trust II Junior Subordinated Deferrable Interest Debentures Issued by FIST II LIBOR	Dec. 31, 2013 First Interstate Statutory Trust I Junior Subordinated Deferrable Interest Debentures Issued by FIST I	Dec. 31, 2012 First Interstate Statutory Trust I Junior Subordinated Deferrable Interest Debentures Issued by FIST I	Nov. 30, 2007 First Interstate Statutory Trust I Junior Subordinated Deferrable Interest Debentures Issued by FIST I	Nov. 30, 2007 First Interstate Statutory Trust I Junior Subordinated Deferrable Interest Debentures Issued by FIST I Debt Instrument, Redemption, At Issuance through Year Five	Nov. 30, 2007 First Interstate Statutory Trust I Junior Subordinated Deferrable Interest Debentures Issued by FIST I LIBOR Debt Instrument, Redemption, After Period Five	Dec. 31, 2013 First Interstate Statutory Trust III Junior Subordinated Deferrable Interest Debentures Issued by FIST III	Dec. 31, 2012 First Interstate Statutory Trust III Junior Subordinated Deferrable Interest Debentures Issued by FIST III	Dec. 31, 2007 First Interstate Statutory Trust III Junior Subordinated Deferrable Interest Debentures Issued by FIST III	Dec. 31, 2007 First Interstate Statutory Trust III Junior Subordinated Deferrable Interest Debentures Issued by FIST III Debt Instrument, Redemption, At Issuance through Year Five	Dec. 31, 2007 First Interstate Statutory Trust III Junior Subordinated Deferrable Interest Debentures Issued by FIST III LIBOR Debt Instrument, Redemption, After Period Five	Dec. 31, 2013 First Interstate Statutory Trust IV Junior Subordinated Deferrable Interest Debentures Issued by FIST IV	Dec. 31, 2012 First Interstate Statutory Trust IV Junior Subordinated Deferrable Interest Debentures Issued by FIST IV	Dec. 31, 2007 First Interstate Statutory Trust IV Junior Subordinated Deferrable Interest Debentures Issued by FIST IV LIBOR	Dec. 31, 2013 First Interstate Statutory Trust V Junior Subordinated Deferrable Interest Debentures Issued by FIST V	Dec. 31, 2012 First Interstate Statutory Trust V Junior Subordinated Deferrable Interest Debentures Issued by FIST V	Jan. 31, 2008 First Interstate Statutory Trust V Junior Subordinated Deferrable Interest Debentures Issued by FIST V	Jan. 31, 2008 First Interstate Statutory Trust V Junior Subordinated Deferrable Interest Debentures Issued by FIST V Debt Instrument, Redemption, At Issuance through Year Five	Jan. 31, 2008 First Interstate Statutory Trust V Junior Subordinated Deferrable Interest Debentures Issued by FIST V LIBOR Debt Instrument, Redemption, After Period Five	Dec. 31, 2013 First Interstate Statutory Trust VI Junior Subordinated Deferrable Interest Debentures Issued by FIST VI	Dec. 31, 2012 First Interstate Statutory Trust VI Junior Subordinated Deferrable Interest Debentures Issued by FIST VI	Jan. 31, 2008 First Interstate Statutory Trust VI Junior Subordinated Deferrable Interest Debentures Issued by FIST VI	Jan. 31, 2008 First Interstate Statutory Trust VI Junior Subordinated Deferrable Interest Debentures Issued by FIST VI LIBOR	Jun. 26, 2012 First Interstate Statutory Trust (FIST) [Member] Junior Subordinated Deferrable Interest Debentures Issued by FIST [Member]	Jun. 26, 2012 First Interstate Statutory Trust (FIST) [Member] Junior Subordinated Deferrable Interest Debentures Issued by FIST [Member] LIBOR
Subordinated Borrowing
Number of company sponsored wholly-owned business trusts	6
Subordinated debentures held by subsidiary trusts	$ 82,477	$ 82,477	$ 10,310	$ 10,310	$ 10,310		$ 15,464	$ 15,464	$ 15,464			$ 20,619	$ 20,619	$ 20,619			$ 15,464	$ 15,464		$ 10,310	$ 10,310	$ 10,310			$ 10,310	$ 10,310	$ 10,310
Redemption price																													41,238
Debt instrument, basis spread on variable rate						2.25%					2.75%					2.40%			2.70%					2.75%				2.75%		3.15%
Liquidation amount per security																													1
Unamortized issuance costs charged to other expense																													428
Mandatory redemption of trust preferred securities																													40,000
Capitalization of subsidiaries																													$ 1,238
Debt instrument, interest rate, stated percentage										7.50%					6.88%								6.78%
Debt instrument, interest rate at period end			2.50%				2.99%					2.64%					2.95%								3.00%