Acquisitions	6 Months Ended
Jun. 30, 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. The acquisition was completed on July 24, 2015 for cash consideration of $7,234. Immediately subsequent to the acquisition, United Bank was merged with and into the Company's existing bank subsidiary, First Interstate Bank ("FIB"). As of the date of the acquisition, Absarokee has total assets of approximately $73,272, loans of approximately $37,520 and deposits of approximately $63,580. 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.

The acquisition was completed on July 24, 2015 for cash consideration of $7,234. Immediately subsequent to the acquisition, United Bank was merged with and into the Company's existing bank subsidiary, First Interstate Bank ("FIB"). As of the date of the acquisition, Absarokee has total assets of approximately $73,272, loans of approximately $37,520 and deposits of approximately $63,580.

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.