Acquisitions	3 Months Ended
Mar. 31, 2015
Business Combinations [Abstract]
Acquisitions	Acquisitions Absarokee Bancorporation, Inc. On March 26, 2015, the Company entered into an agreement and plan of merger to acquire all of the outstanding stock of Absarokee Bancorporation, Inc. ("Absarokee"), a Montana-based bank holding company that operates one subsidiary bank, United Bank, with branches located in three Montana communities adjacent to the Company's existing market areas. As of March 31, 2015, Absarokee has total assets of approximately $74 million, net loans of approximately $37 million and deposits of approximately $64 million. Under the terms of the agreement and plan of merger, the Company will pay cash consideration of approximately $7,234, subject to certain financial performance and other adjustments, the amount of which will be determined prior to the closing date of the transaction. The merger is expected to be completed during third quarter 2015. Subject to regulatory approval, the Company anticipates United Bank will be merged with and into its bank subsidiary, First Interstate Bank ("FIB"), immediately subsequent to the merger. Mountain West Financial Corp. On July 31, 2014, the Company acquired all of the outstanding stock of Mountain West Financial Corp ("MWFC"), a Montana-based bank holding company operating one subsidiary bank, Mountain West Bank, NA ("MWB"). MWB was merged with and into FIB in October 2014. During March 2015, the Company completed its review of MWFC tax items and finalized the fair value of acquired deferred tax assets. Finalization of provisional estimates resulted in a $1,199 decrease in goodwill.

Acquisitions

Absarokee Bancorporation, Inc. On March 26, 2015, the Company entered into an agreement and plan of merger to acquire all of the outstanding stock of Absarokee Bancorporation, Inc. ("Absarokee"), a Montana-based bank holding company that operates one subsidiary bank, United Bank, with branches located in three Montana communities adjacent to the Company's existing market areas. As of March 31, 2015, Absarokee has total assets of approximately $74 million, net loans of approximately $37 million and deposits of approximately $64 million.

Under the terms of the agreement and plan of merger, the Company will pay cash consideration of approximately $7,234, subject to certain financial performance and other adjustments, the amount of which will be determined prior to the closing date of the transaction. The merger is expected to be completed during third quarter 2015. Subject to regulatory approval, the Company anticipates United Bank will be merged with and into its bank subsidiary, First Interstate Bank ("FIB"), immediately subsequent to the merger.

Mountain West Financial Corp. On July 31, 2014, the Company acquired all of the outstanding stock of Mountain West Financial Corp ("MWFC"), a Montana-based bank holding company operating one subsidiary bank, Mountain West Bank, NA ("MWB"). MWB was merged with and into FIB in October 2014. During March 2015, the Company completed its review of MWFC tax items and finalized the fair value of acquired deferred tax assets. Finalization of provisional estimates resulted in a $1,199 decrease in goodwill.