Segment Reporting - Segment Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
Segment Reporting Information [Line Items]
Net interest income	$ 111,384	$ 88,476	$ 301,737	$ 257,615
Provision for credit losses	11,367	(2,531)	19,438	(30,224)
Mortgage banking income	16,729	47,751	69,718	146,990
Change in fair value of mortgage servicing rights, net of hedging	(4,345)	(2,367)	(5,244)	(10,775)
Other noninterest income	10,208	13,622	32,724	38,821
Depreciation and amortization	2,057	2,140	6,105	6,288
Amortization of intangibles	1,108	1,344	3,546	4,178
Other noninterest expense	78,682	91,523	258,465	272,199
Income before income taxes	40,762	55,006	111,381	180,210
Income tax expense	8,931	9,716	24,961	38,744
Net income applicable to FB Financial Corporation and noncontrolling interest	31,831	45,290	86,420	141,466
Net income applicable to noncontrolling interest	0	0	8	8
Net income applicable to FB Financial Corporation	31,831	45,290	86,412	141,458
Assets	12,258,082	11,810,290	12,258,082	11,810,290	$ 12,597,686
Goodwill	242,561	242,561	242,561	242,561	$ 242,561
Provision (reversal) for credit losses on unfunded commitments	3,178	301	9,197	(2,875)
Mortgage restructuring expense	0	0	12,458	0
Banking(4)
Segment Reporting Information [Line Items]
Net interest income	111,384	88,576	301,739	257,726
Provision for credit losses	11,367	(2,531)	19,438	(30,224)
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	10,293	13,823	32,975	39,223
Depreciation and amortization	1,867	1,791	5,308	5,267
Amortization of intangibles	1,108	1,344	3,546	4,178
Other noninterest expense	62,911	55,642	175,936	163,261
Income before income taxes	44,424	46,153	130,486	154,467
Assets	11,648,610	10,712,281	11,648,610	10,712,281
Goodwill	242,561	242,561	242,561	242,561
Mortgage
Segment Reporting Information [Line Items]
Net interest income	0	(100)	(2)	(111)
Provision for credit losses	0	0	0	0
Mortgage banking income	16,729	47,751	69,718	146,990
Change in fair value of mortgage servicing rights, net of hedging	(4,345)	(2,367)	(5,244)	(10,775)
Other noninterest income	(85)	(201)	(251)	(402)
Depreciation and amortization	190	349	797	1,021
Amortization of intangibles	0	0	0	0
Other noninterest expense	15,771	35,881	82,529	108,938
Income before income taxes	(3,662)	8,853	(19,105)	25,743
Assets	609,472	1,098,009	609,472	1,098,009
Goodwill	$ 0	$ 0	$ 0	$ 0