Merger with Pioneer Bancshares, Inc. - Narrative (Details) - FirstSun Capital Bancorp and Pioneer Bancshares, Inc. Merger $ in Thousands	Apr. 01, 2022 USD ($) shares
Business Acquisition [Line Items]
Business acquisition, shares issuable in merger (in shares) | shares	1.0443
Business acquisition, number of shares issued (in shares) | shares	6,467,466
Business acquisition, equity interest issued	$ 230,760
Business acquisition, consideration transferred, stock options converted, number of shares (in shares) | shares	431,645
Business acquisition, consideration transferred, stock options converted	$ 5,334
Payments to acquire businesses	4,736
Business acquisition, aggregate consideration	$ 240,830