Segment Reporting - Schedule of Segment Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Segment Reporting Information [Line Items]
Interest income	$ 236,898	$ 185,628	$ 598,688	$ 539,169
Interest expense	89,658	79,611	232,392	231,047
Net interest income	147,240	106,017	366,296	308,122
Provisions for credit losses	34,417	1,914	42,046	4,920
Net interest income after provision for credit losses	112,823	104,103	324,250	303,202
Mortgage banking income	16,874	16,043	49,629	47,915
Change in fair value of mortgage servicing rights, net of hedging	(3,390)	(4,490)	(10,690)	(11,867)
Other noninterest income	13,151	(28,050)	(23,824)	(18,975)
Total noninterest income	26,635	(16,497)	15,115	17,073
Salaries, commissions and employee benefits	59,210	47,538	154,192	138,381
Merger and integration costs	16,057	0	19,192	0
Depreciation and amortization	3,185	3,255	8,820	8,957
Amortization of intangibles	2,079	719	3,366	2,260
Other noninterest expense	29,325	24,700	85,096	74,127
Total noninterest expense	109,856	76,212	270,666	223,725
Income before income taxes	29,602	11,394	68,699	96,550
Income tax expense	6,227	1,174	3,046	18,393
Net income applicable to FB Financial Corporation and noncontrolling interest	23,375	10,220	65,653	78,157
Net income applicable to noncontrolling interest	0	0	8	8
Net income applicable to FB Financial Corporation	23,375	10,220	65,645	78,149
Total assets	16,236,459	12,920,222	16,236,459	12,920,222	$ 13,157,482
Goodwill	350,353	242,561	350,353	242,561	$ 242,561
Banking
Segment Reporting Information [Line Items]
Interest income	236,073	185,824	595,948	539,814
Interest expense	91,214	81,489	236,421	235,824
Net interest income	144,859	104,335	359,527	303,990
Provisions for credit losses	34,070	1,861	36,841	5,131
Net interest income after provision for credit losses	110,789	102,474	322,686	298,859
Mortgage banking income	0	0	0	0
Change in fair value of mortgage servicing rights, net of hedging	0	0	0	0
Other noninterest income	13,078	(28,370)	(23,982)	(19,687)
Total noninterest income	13,078	(28,370)	(23,982)	(19,687)
Salaries, commissions and employee benefits	51,441	39,938	131,545	116,521
Merger and integration costs	16,057		19,192
Depreciation and amortization	3,167	3,141	8,759	8,594
Amortization of intangibles	2,079	719	3,366	2,260
Other noninterest expense	24,225	19,316	68,346	58,111
Total noninterest expense	96,969	63,114	231,208	185,486
Income before income taxes	26,898	10,990	67,496	93,686
Total assets	15,598,629	12,337,135	15,598,629	12,337,135
Goodwill	350,353	242,561	350,353	242,561
Mortgage
Segment Reporting Information [Line Items]
Interest income	825	(196)	2,740	(645)
Interest expense	(1,556)	(1,878)	(4,029)	(4,777)
Net interest income	2,381	1,682	6,769	4,132
Provisions for credit losses	347	53	5,205	(211)
Net interest income after provision for credit losses	2,034	1,629	1,564	4,343
Mortgage banking income	16,874	16,043	49,629	47,915
Change in fair value of mortgage servicing rights, net of hedging	(3,390)	(4,490)	(10,690)	(11,867)
Other noninterest income	73	320	158	712
Total noninterest income	13,557	11,873	39,097	36,760
Salaries, commissions and employee benefits	7,769	7,600	22,647	21,860
Merger and integration costs	0		0
Depreciation and amortization	18	114	61	363
Amortization of intangibles	0	0	0	0
Other noninterest expense	5,100	5,384	16,750	16,016
Total noninterest expense	12,887	13,098	39,458	38,239
Income before income taxes	2,704	404	1,203	2,864
Total assets	637,830	583,087	637,830	583,087
Goodwill	$ 0	$ 0	$ 0	$ 0