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Stockholders' Equity - Narrative (Details)	1 Months Ended		3 Months Ended
Stockholders' Equity - Narrative (Details)	Feb. 28, 2025 USD ($) shares	Mar. 31, 2024 USD ($) shares	Mar. 31, 2025 USD ($) company $ / shares shares	Mar. 31, 2024 USD ($)	Dec. 31, 2024 $ / shares shares	Dec. 31, 2021 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred stock, shares authorized (in shares)			10,000,000		10,000,000
Preferred stock, par value (in dollars per share) \| $ / shares			$ 0.0001		$ 0.0001
Preferred stock, shares issued (in shares)			0		0
Preferred stock, shares outstanding (in shares)			0		0
Common stock, shares authorized (in shares)			50,000,000		50,000,000
Common stock, par value (in dollars per share) \| $ / shares			$ 0.0001		$ 0.0001
Common stock, shares issued (in shares)			27,753,918		27,709,679
Common stock, shares outstanding (in shares)			27,753,918		27,709,679
Dividends, term without prior regulatory approval, preceding period of net income			2 years
Payments of dividends \| $			$ 0	$ 0
Option awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award contractual term			10 years
Option awards \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, number of comparable companies \| company			25
Option awards \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, number of comparable companies \| company			30
Option awards \| Second Anniversaries
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage			25.00%
Option awards \| Third Anniversaries
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage			25.00%
Option awards \| Fourth Anniversaries
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage			25.00%
Option awards \| First Anniversaries
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage			25.00%
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation cost related to non-vested stock options granted \| $			$ 0
Share-based payment arrangement, compensation cost \| $	$ 184,000
Performance-based restricted stock \| April 2024
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation cost related to non-vested stock options granted \| $			$ 1,992,000
Awards expected to issue (in shares)			84,150
Performance-based restricted stock \| April 2023
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation cost related to non-vested stock options granted \| $			$ 727,000
Awards expected to issue (in shares)			77,873
Performance-based restricted stock \| April 2022
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation cost related to non-vested stock options granted \| $			$ 0
Awards expected to issue (in shares)			42,403
FirstSun Capital Bancorp 2017 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate shares of common stock (in shares)			1,977,292
Total unrecognized compensation cost related to non-vested stock options granted \| $			$ 9,000
Intrinsic value of the stock options \| $		$ 17,439,000	14,089,000	17,439,000
FirstSun Capital Bancorp 2021 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate shares of common stock (in shares)						2,476,571
Issuance of common stock on restricted stock grants, net of forfeitures (in shares)	2,997	11,739
FirstSun Capital Bancorp 2017 And 2021 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment arrangement, compensation cost \| $			636,000	492,000
Pioneer's Option Plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Intrinsic value of the stock options \| $		$ 1,243,000	1,167,000	$ 1,243,000
Bank
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Payments of dividends \| $			$ 75,000