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Stockholders' Equity - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment arrangement, compensation cost	$ 348	$ 526	$ 1,501	$ 1,752
Total unrecognized compensation cost related to non-vested stock options granted	1,390		1,390
Intrinsic value of the stock options	$ 15,343		$ 15,343		$ 10,660
Share-based Payment Arrangement, Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation cost related to non-vested stock options granted, period			3 years 8 months 1 day
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		1,977,292