Acquisitions	6 Months Ended
Jun. 30, 2016
Business Combinations [Abstract]
Acquisitions	Acquisitions Flathead Bank. On April 6, 2016, the Company's bank subsidiary, First Interstate Bank, entered into a stock purchase agreement to acquire all of the outstanding stock of Flathead Bank of Bigfork ("Flathead Bank"), a Montana-based bank wholly owned by Flathead Holding Company. With total assets of $231,574 as of December 31, 2015, Flathead Bank operates seven branches in western and northwestern Montana. Upon closing of the transaction, which is expected to occur during the third quarter of 2016, all Flathead Bank branches will become branches of First Interstate Bank. Pursuant to the Purchase Agreement, First Interstate Bank will pay cash consideration of approximately $34,237 for the stock, subject to certain financial performance and other adjustments, the amount of which will be determined prior to the closing date of the transaction. Absarokee Bancorporation, Inc. On July 24, 2015, the Company acquired all of the outstanding stock of Absarokee Bancorporation, Inc. ("Absarokee"), a Montana-based bank holding company that operated one subsidiary bank, United Bank. The Company merged United Bank with and into First Interstate Bank immediately subsequent to the acquisition. During March 2016, the Company completed its review of Absarokee's tax items and finalized the fair value of the acquired deferred tax asset. Finalization of provisional estimates resulted in a $42 decrease in goodwill.

Acquisitions

Flathead Bank. On April 6, 2016, the Company's bank subsidiary, First Interstate Bank, entered into a stock purchase agreement to acquire all of the outstanding stock of Flathead Bank of Bigfork ("Flathead Bank"), a Montana-based bank wholly owned by Flathead Holding Company. With total assets of $231,574 as of December 31, 2015, Flathead Bank operates seven branches in western and northwestern Montana. Upon closing of the transaction, which is expected to occur during the third quarter of 2016, all Flathead Bank branches will become branches of First Interstate Bank. Pursuant to the Purchase Agreement, First Interstate Bank will pay cash consideration of approximately $34,237 for the stock, subject to certain financial performance and other adjustments, the amount of which will be determined prior to the closing date of the transaction.

Absarokee Bancorporation, Inc. On July 24, 2015, the Company acquired all of the outstanding stock of Absarokee Bancorporation, Inc. ("Absarokee"), a Montana-based bank holding company that operated one subsidiary bank, United Bank. The Company merged United Bank with and into First Interstate Bank immediately subsequent to the acquisition. During March 2016, the Company completed its review of Absarokee's tax items and finalized the fair value of the acquired deferred tax asset. Finalization of provisional estimates resulted in a $42 decrease in goodwill.