Cover - shares	6 Months Ended
	Jun. 30, 2020	Aug. 07, 2020
Cover [Abstract]
Document Quarterly Report	true
Document Transition Report	false
Document Type	10-Q
Document Period End Date	Jun. 30, 2020
Entity File Number	001-37621
Entity Registrant Name	FIRST GUARANTY BANCSHARES, INC.
Entity Incorporation, State or Country Code	LA
Entity Tax Identification Number	26-0513559
Entity Address, Address Line One	400 East Thomas Street
Entity Address, City or Town	Hammond,
Entity Address, State or Province	LA
Entity Address, Postal Zip Code	70401
City Area Code	(985)
Local Phone Number	345-7685
Title of 12(b) Security	Common Stock, $1 par value
Trading Symbol	FGBI
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		9,741,253
Entity Central Index Key	0001408534
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Amendment Flag	false