Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2023
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events The Company has evaluated subsequent events for potential recognition and disclosure through the filing date of this Form 10-K. Proposed Merger with HomeStreet On January 16, 2024, FirstSun entered into an Agreement and Plan of Merger, that provides for the combination of FirstSun and HomeStreet. Under the merger agreement, a newly formed wholly-owned subsidiary of FirstSun, will merge with and into HomeStreet, with HomeStreet remaining as the surviving entity and becoming a wholly-owned subsidiary of FirstSun, with FirstSun as the surviving entity, in a transaction we refer to as the “first step merger.” This surviving entity, immediately following the first step merger and as part of a single integrated transaction, will merge with and into FirstSun, in a transaction we refer to as the “second step merger.” Immediately following the completion of the second step merger, HomeStreet Bank will merge with and into Sunflower Bank. Upon the terms and subject to the conditions set forth in the merger agreement, at the effective time of the merger, each share of common stock of HomeStreet issued and outstanding immediately prior to the merger effective time, subject to certain exceptions, will be converted into the right to receive 0.4345 of a share of FirstSun common stock. Holders of such HomeStreet common stock, subject to certain exceptions, will also be entitled to receive cash in lieu of fractional shares of FirstSun common stock. Consummation of the proposed HomeStreet merger is subject to the satisfaction of customary closing conditions including receipt of necessary HomeStreet shareholder and regulatory approvals. The merger agreement provides certain termination rights for both FirstSun and HomeStreet and further provides that a termination fee will be payable by either FirstSun or HomeStreet, as applicable, upon termination of the merger agreement under certain circumstances. The parties to the merger expect to close the merger in the middle of 2024, subject to satisfaction of closing conditions, including receipt of customary required regulatory approvals and the requisite approval by the shareholders of HomeStreet. The combined entity is expected to have total assets of approximately $17 billion and 129 branch locations. The combined entity’s expanded footprint includes, FirstSun’s current presence in the Southwest and Midwest together with HomeStreet’s presence in Southern California, Hawaii and the Pacific Northwest. Investment Agreements Upfront Securities Purchase Agreement Concurrently with entry into the merger agreement, FirstSun entered into an upfront securities purchase agreement (the “Upfront Securities Purchase Agreement”) with certain funds managed by Wellington Management Company, LLP (collectively, the “Wellington Funds”), pursuant to which we issued 2,461,538 shares of our common stock in a private placement for $80.0 million that closed on January 17, 2024. Under the terms of the Upfront Securities Purchase Agreement, FirstSun is also obligated, concurrently with the closing of the proposed HomeStreet merger, to issue to the Wellington Funds, warrants (the “Warrants”) to purchase approximately 1.15 million shares of FirstSun common stock with such Warrants having an initial exercise price of $32.50 per share. The Warrants will carry a term of three years. In the event the proposed HomeStreet merger is not consummated, no Warrants will be issued. Acquisition Finance Securities Purchase Agreement Concurrently with its entry into the HomeStreet merger agreement, FirstSun entered into an acquisition finance securities purchase agreement (the “Acquisition Finance Securities Purchase Agreement,” and together with the Upfront Securities Purchase Agreement, as the “Investment Agreements”), dated January 16, 2024, with the Wellington Funds and certain other institutional investors (who we refer to as “additional institutional investors” and, collectively with the Wellington Funds, as the “institutional investors”). Pursuant to the Acquisition Finance Securities Purchase Agreement, on the terms and subject to the conditions set forth therein, substantially concurrently with the closing of the merger, the institutional investors will invest an aggregate of $95 million in exchange for the sale and issuance, at a purchase price of $32.50 per share, of approximately 2.92 million shares of FirstSun common stock. Registration Rights Agreements In connection with the Upfront Securities Purchase Agreement, FirstSun and the Wellington Funds also entered into a registration rights agreement (the “Upfront Registration Rights Agreement”), dated January 16, 2024, pursuant to which FirstSun agreed to, among other things, provide customary resale registration rights with respect to the shares of our common stock obtained by the Wellington Funds pursuant to the Investment Agreements, including those issued upon exercise of the Warrants. In addition. the Acquisition Finance Securities Purchase Agreement contemplates that, in connection with the closing of the investments under the Acquisition Finance Securities Purchase Agreement, FirstSun will enter into a resale registration rights agreement with each additional institutional investor (the “Acquisition Finance Registration Rights Agreement”), the material terms and conditions of which are consistent with the terms and conditions of the Upfront Registration Rights Agreement. Charter Amendment In connection with the proposed merger, the holders of a majority of the voting power of FirstSun common stock executed a written consent approving and adopting an amendment to our certificate of incorporation (the “Charter Amendment”) which will increase the number of FirstSun’s authorized shares of capital stock from 60,000,000 to 110,000,000, consisting of 100,000,000 shares of FirstSun common stock, and 10,000,000 shares of preferred stock and will become effective upon FirstSun’s filing the Charter Amendment with the Secretary of State of the State of Delaware. We plan to file the Charter Amendment prior to the closing of the proposed merger.