Segment reporting - Schedule of Segment Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Net interest income	$ 68,828	$ 58,305	$ 180,414	$ 168,344
Provisions for credit losses	55,401	1,831	110,887	4,103
Mortgage banking income	92,126	34,284	216,145	85,512
Change in fair value of mortgage servicing rights, net of hedging	(7,440)	(5,091)	(26,546)	(10,772)
Other noninterest income	12,340	8,952	31,618	25,423
Depreciation and amortization	1,693	1,380	4,924	3,830
Amortization of intangibles	1,417	1,197	3,825	3,180
Other noninterest mortgage banking expense	45,084	23,648	108,129	65,698
Other noninterest expense	69,898	36,710	150,352	109,447
(Loss) income before income taxes	(7,639)	31,684	23,514	82,249
Income tax (benefit) expense	(2,040)	7,718	5,495	20,007
Net (loss) income applicable to FB Financial Corporation	(5,599)	23,966	18,019	62,242
Total assets	11,010,438	6,088,895	11,010,438	6,088,895	$ 6,124,921
Goodwill	236,086	168,486	236,086	168,486	$ 169,051
Provision for credit losses on unfunded commitments	9,567	0	13,050	0
Merger costs	20,730	295	25,366	4,699
Banking Segment
Segment Reporting Information [Line Items]
Net interest income	68,791	58,350	180,374	168,322
Provisions for credit losses	55,401	1,831	110,887	4,103
Mortgage banking income	24,683	10,693	52,274	20,530
Change in fair value of mortgage servicing rights, net of hedging	0	0	0	0
Other noninterest income	12,340	8,952	31,618	25,423
Depreciation and amortization	1,550	1,255	4,545	3,431
Amortization of intangibles	1,417	1,197	3,825	3,180
Other noninterest mortgage banking expense	15,175	8,087	33,892	15,090
Other noninterest expense	69,568	36,598	150,022	107,452
(Loss) income before income taxes	(37,297)	29,027	(38,905)	81,019
Total assets	10,378,122	5,730,492	10,378,122	5,730,492
Goodwill	236,086	168,486	236,086	168,486
Merger costs	20,400	295	25,036
Mortgage Segment
Segment Reporting Information [Line Items]
Net interest income	37	(45)	40	22
Provisions for credit losses	0	0	0	0
Mortgage banking income	67,443	23,591	163,871	64,982
Change in fair value of mortgage servicing rights, net of hedging	(7,440)	(5,091)	(26,546)	(10,772)
Other noninterest income	0	0	0	0
Depreciation and amortization	143	125	379	399
Amortization of intangibles	0	0	0	0
Other noninterest mortgage banking expense	29,909	15,561	74,237	50,608
Other noninterest expense	330	112	330	1,995
(Loss) income before income taxes	29,658	2,657	62,419	1,230
Total assets	632,316	358,403	632,316	358,403
Goodwill	0	$ 0	0	$ 0
Merger costs	$ 330		$ 330