Condensed Parent Company Information, Condensed Statements of Income (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating Income [Abstract]
Net gains on securities	$ 2,700	$ 3,300	$ 295	$ 1,571
Operating Expenses [Abstract]
Interest expense		8,608	9,202	11,134
Salaries & Benefits		15,496	15,840	14,368
Income Before Income Taxes		21,468	16,716	13,723
Income tax benefit (expense)		(6,963)	(5,492)	(4,577)
Net Income		14,505	11,224	9,146
Less preferred stock dividends		(384)	(394)	(713)
Income Available to Common Shareholders		14,121	10,830	8,433
First Guaranty Bancshares, Inc. [Member]
Operating Income [Abstract]
Dividends received from bank subsidiary		9,843	6,448	4,669
Net gains on securities		2,652	0	0
Other income		261	162	90
Total operating income		12,756	6,610	4,759
Operating Expenses [Abstract]
Interest expense		192	130	115
Salaries & Benefits		172	140	88
Other expenses		766	464	449
Total operating expenses		1,130	734	652
Income Before Income Taxes		11,626	5,876	4,107
Income tax benefit (expense)		(605)	229	212
Income before increase in equity in undistributed earnings of subsidiary		11,021	6,105	4,319
Increase in equity in undistributed earnings of subsidiary		3,484	5,119	4,827
Net Income		14,505	11,224	9,146
Less preferred stock dividends		(384)	(394)	(713)
Income Available to Common Shareholders		$ 14,121	$ 10,830	$ 8,433