Segment reporting - Segment Financial Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Net interest income	$ 347,370	$ 265,658	$ 226,036
Provision for credit losses	(40,993)	107,967	7,053
Mortgage banking income	179,682	289,702	117,905
Change in fair value of mortgage servicing rights, net of hedging	(12,117)	(34,374)	(16,989)
Other noninterest income	60,690	46,527	34,481
Depreciation and amortization	8,416	7,536	5,176
Amortization of intangibles	5,473	5,323	4,339
Other noninterest expense	359,678	364,226	235,326
Income before income taxes	243,051	82,461	109,539
Income tax expense	52,750	18,832	25,725
Net income applicable to FB Financial Corporation and noncontrolling interest	190,301	63,629	83,814
Net income applicable to noncontrolling interest	16	8	0
Net income applicable to FB Financial Corporation	190,285	63,621	83,814
Assets	12,597,686	11,207,330	6,124,921
Goodwill	242,561	242,561	169,051
Provision (reversal) for credit losses on unfunded commitments	(1,998)	13,361	0
Merger costs	0	34,879	5,385
Franklin Financial Network, Inc.
Segment Reporting Information [Line Items]
Other noninterest income	11,172	3,228
Banking
Segment Reporting Information [Line Items]
Net interest income	347,342	265,581	226,098
Provision for credit losses	(40,993)	107,967	7,053
Mortgage banking income	0	0	0
Change in fair value of mortgage servicing rights, net of hedging	0	0	0
Other noninterest income	61,073	46,527	34,481
Depreciation and amortization	7,054	6,425	4,594
Amortization of intangibles	5,473	5,323	4,339
Other noninterest expense	220,283	212,890	144,734
Income before income taxes	216,598	(20,497)	99,859
Assets	11,540,560	10,254,324	5,700,558
Goodwill	242,561	242,561	169,051
Banking | Farmers National
Segment Reporting Information [Line Items]
Merger costs		33,824
Banking | Atlantic Capital
Segment Reporting Information [Line Items]
Merger costs			5,385
Mortgage
Segment Reporting Information [Line Items]
Net interest income	28	77	(62)
Provision for credit losses	0	0	0
Mortgage banking income	179,682	289,702	117,905
Change in fair value of mortgage servicing rights, net of hedging	(12,117)	(34,374)	(16,989)
Other noninterest income	(383)	0	0
Depreciation and amortization	1,362	1,111	582
Amortization of intangibles	0	0	0
Other noninterest expense	139,395	151,336	90,592
Income before income taxes	26,453	102,958	9,680
Assets	1,057,126	953,006	424,363
Goodwill	$ 0	0	0
Mortgage | Franklin Financial Network, Inc.
Segment Reporting Information [Line Items]
Merger costs		$ 1,055
Mortgage | Atlantic Capital
Segment Reporting Information [Line Items]
Other noninterest expense			$ 1,995