Subordinated Debentures Held by Subsidiary Trusts (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended		0 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 26, 2012 Junior Subordinated Deferrable Interest Debentures Issued by FIST [Member]	Jun. 26, 2012 First Interstate Statutory Trust I (FIST) [Member] Junior Subordinated Deferrable Interest Debentures Issued by FIST [Member]
Subordinated Borrowing [Line Items]
Redemption Price			$ 41,238
Liquidation Amount Per Security			1
Unamortized Issuance Costs Charged to Other Expense			428
Mandatory Redemption of Trust Preferred Securities			40,000
Capital distribution by unconsolidated subsidiary trust	$ 1,238	$ 0		$ 1,238