Segment Reporting - Segment Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021
Segment Reporting Information [Line Items]
Net interest income	$ 102,171	$ 86,563	$ 190,353	$ 169,139
Provision for credit losses	12,318	(13,839)	8,071	(27,693)
Mortgage banking income	23,711	38,644	52,989	99,239
Change in fair value of mortgage servicing rights, net of hedging	(1,152)	(3,145)	(899)	(8,408)
Other noninterest income	10,655	13,801	22,516	25,199
Depreciation and amortization	2,012	1,962	4,048	4,148
Amortization of intangibles	1,194	1,394	2,438	2,834
Other noninterest expense	93,791	89,604	179,783	180,676
Income before income taxes	26,070	56,742	70,619	125,204
Income tax expense	6,717	13,440	16,030	29,028
Net income applicable to FB Financial Corporation and noncontrolling interest	19,353	43,302	54,589	96,176
Net income applicable to noncontrolling interest	8	8	8	8
Net income applicable to FB Financial Corporation	19,345	43,294	54,581	96,168
Assets	12,193,862	11,918,367	12,193,862	11,918,367	$ 12,597,686
Goodwill	242,561	242,561	242,561	242,561	$ 242,561
Provision (reversal) for credit losses on unfunded commitments	4,137	(954)	6,019	(3,176)
Mortgage restructuring expense	12,458	0	12,458	0
Banking(4)
Segment Reporting Information [Line Items]
Net interest income	102,171	86,553	190,355	169,150
Provision for credit losses	12,318	(13,839)	8,071	(27,693)
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	10,699	14,002	22,682	25,400
Depreciation and amortization	1,731	1,618	3,441	3,476
Amortization of intangibles	1,194	1,394	2,438	2,834
Other noninterest expense	56,395	55,182	113,025	107,619
Income before income taxes	41,232	56,200	86,062	108,314
Assets	11,469,762	10,908,107	11,469,762	10,908,107
Goodwill	242,561	242,561	242,561	242,561
Mortgage
Segment Reporting Information [Line Items]
Net interest income	0	10	(2)	(11)
Provision for credit losses	0	0	0	0
Mortgage banking income	23,711	38,644	52,989	99,239
Change in fair value of mortgage servicing rights, net of hedging	(1,152)	(3,145)	(899)	(8,408)
Other noninterest income	(44)	(201)	(166)	(201)
Depreciation and amortization	281	344	607	672
Amortization of intangibles	0	0	0	0
Other noninterest expense	37,396	34,422	66,758	73,057
Income before income taxes	(15,162)	542	(15,443)	16,890
Assets	724,100	1,010,260	724,100	1,010,260
Goodwill	0	$ 0	0	$ 0
Mortgage restructuring expense	$ 12,458		$ 12,458