Segment reporting - Segment Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Net interest income	$ 82,576	$ 56,249
Provision for credit losses	(13,854)	29,565
Mortgage banking income	60,595	38,613
Change in fair value of mortgage servicing rights, net of hedging	(5,263)	(5,868)
Other noninterest income	11,398	9,955
Depreciation and amortization	2,056	1,612
Amortization of intangibles	1,440	1,203
Other noninterest expense	91,202	65,744
Income before income taxes	68,462	825
Income tax expense	15,588	80
Net income applicable to FB Financial Corporation and noncontrolling interest	52,874	745
Net income applicable to noncontrolling interest	0	0
Net income applicable to FB Financial Corporation	52,874	745
Assets	11,935,826	6,655,687	$ 11,207,330
Goodwill	242,561	174,859	$ 242,561
(Reversal) provision for credit losses on unfunded commitments	(2,222)	1,601
Merger costs	0	3,050
Banking
Segment Reporting Information [Line Items]
Net interest income	82,597	56,233
Provision for credit losses	(13,854)	29,565
Mortgage banking income	0	0
Change in fair value of mortgage servicing rights, net of hedging	0	0
Other noninterest income	11,398	9,955
Depreciation and amortization	1,858	1,492
Amortization of intangibles	1,440	1,203
Other noninterest expense	52,437	41,122
Income before income taxes	52,114	(7,194)
Assets	10,787,955	6,122,217
Goodwill	242,561	174,859
Merger costs		3,050
Mortgage
Segment Reporting Information [Line Items]
Net interest income	(21)	16
Provision for credit losses	0	0
Mortgage banking income	60,595	38,613
Change in fair value of mortgage servicing rights, net of hedging	(5,263)	(5,868)
Other noninterest income	0	0
Depreciation and amortization	198	120
Amortization of intangibles	0	0
Other noninterest expense	38,765	24,622
Income before income taxes	16,348	8,019
Assets	1,147,871	533,470
Goodwill	$ 0	$ 0