Acquisitions	3 Months Ended
Mar. 31, 2014
Business Combinations [Abstract]
Acquisitions	Acquisitions On February 10, 2014, the Company entered into an agreement and plan of merger to acquire all of the outstanding stock of Mountain West Financial Corp ("Mountain West"), a Montana-based bank holding company that operates one wholly-owned subsidiary bank, Mountain West Bank, NA, with branches located in five of the Company's current market areas in Montana. As of December 31, 2013, Mountain West had loans of approximately $405,409 and deposits of approximately $539,920. Under the terms of the agreement and plan of merger, each outstanding share of Mountain West common stock will be canceled and converted into the right to receive 0.2552 shares of the Company's Class A common stock plus $7.125 in cash, or, if a Mountain West stockholder properly elects and subject to the limitations contained within the agreement and plan of merger, an amount in all cash or all stock intended to be substantially equal in value to the above described combination of stock and cash merger consideration. The value received by Mountain West stockholders in the aggregate and on a per share of Mountain West common stock basis will fluctuate prior to the completion of the merger based on the prevailing market price of the Company's Class A common stock at the time the transaction is consummated. Based on the closing price of the Company's Class A common stock on the NASDAQ Global Select Market on March 31, 2014 of $28.22 per share, and on the number of shares and options to purchase shares of Mountain West common stock issued and outstanding on March 31, 2014, the aggregate value of the consideration expected to be paid would be $78,814, of which $39,920 would be paid in cash and $38,894 would be represented by 1,378,230 shares of the Company's Class A common stock. The merger is expected to close in mid-2014, subject to several conditions, including, among other things, receipt of applicable regulatory approval and approval by the stockholders of Mountain West. The Company anticipates that within six months following the consummation of the merger, Mountain West Bank, NA will be merged with and into FIB.

Acquisitions

On February 10, 2014, the Company entered into an agreement and plan of merger to acquire all of the outstanding stock of Mountain West Financial Corp ("Mountain West"), a Montana-based bank holding company that operates one wholly-owned subsidiary bank, Mountain West Bank, NA, with branches located in five of the Company's current market areas in Montana. As of December 31, 2013, Mountain West had loans of approximately $405,409 and deposits of approximately $539,920.

Under the terms of the agreement and plan of merger, each outstanding share of Mountain West common stock will be canceled and converted into the right to receive 0.2552 shares of the Company's Class A common stock plus $7.125 in cash, or, if a Mountain West stockholder properly elects and subject to the limitations contained within the agreement and plan of merger, an amount in all cash or all stock intended to be substantially equal in value to the above described combination of stock and cash merger consideration. The value received by Mountain West stockholders in the aggregate and on a per share of Mountain West common stock basis will fluctuate prior to the completion of the merger based on the prevailing market price of the Company's Class A common stock at the time the transaction is consummated. Based on the closing price of the Company's Class A common stock on the NASDAQ Global Select Market on March 31, 2014 of $28.22 per share, and on the number of shares and options to purchase shares of Mountain West common stock issued and outstanding on March 31, 2014, the aggregate value of the consideration expected to be paid would be $78,814, of which $39,920 would be paid in cash and $38,894 would be represented by 1,378,230 shares of the Company's Class A common stock.

The merger is expected to close in mid-2014, subject to several conditions, including, among other things, receipt of applicable regulatory approval and approval by the stockholders of Mountain West. The Company anticipates that within six months following the consummation of the merger, Mountain West Bank, NA will be merged with and into FIB.