Capital Stock

6 Months Ended

Jun. 30, 2026

Equity [Abstract]

Capital Stock

The Company’s common stock is traded on the NASDAQ stock market under the symbol “FIBK.”

As of June 30, 2026, the Company is authorized to issue an aggregate of 150,100,000 shares of capital stock, of which 150,000,000 shares are designated as common stock, and 100,000 are designated as preferred stock. Our common stock is uncertificated and has one vote per share.

The Company had 95,548,084 shares and 101,105,745 shares of common stock outstanding as of June 30, 2026 and December 31, 2025, respectively, and no shares of preferred stock outstanding as of June 30, 2026 and December 31, 2025.

On August 28, 2025, the board of directors of the Company adopted a new stock repurchase program, pursuant to which the Company has been authorized to repurchase up to $150.0 million worth of its issued and outstanding shares of common stock on or prior to March 31, 2027, which is the expiration date of the program. On January 27, 2026, the board of directors authorized an increase to the repurchase program of an additional $150.0 million, and on July 22, 2026 the board of directors authorized a further increase to the repurchase program of an additional $150.0 million, bringing the total repurchase authorization since August 2025 to $450.0 million. Any repurchased shares will be returned to authorized but unissued shares of common stock, as permitted under applicable Delaware law.

During the three months ended June 30, 2026, 1,929,769 shares of common stock were repurchased under the stock repurchase program at a total cost of $68.7 million or at a weighted-average price of $35.61 per share. During the six months ended June 30, 2026, 4,322,662 shares of common stock were repurchased under the stock repurchase program at a total cost of $152.7 million or at a weighted-average price of $35.32 per share. As of June 30, 2026, following these repurchases, approximately $29.7 million remained available for future purchases under the program. The additional stock repurchases during the three and the six months ended June 30, 2026 and 2025, were redemptions of vested restricted shares tendered in lieu of cash for payment of income tax withholding amounts by participants in the Company’s equity compensation plans.