Capital Requirements - Actual Capital Amounts and Ratios (Details) $ in Thousands	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Tier 1 Leverage Capital, Amount
Actual	$ 10,000,000
First Guaranty Bank
Total Risk-based Capital, Amount
Actual	330,944	$ 308,510
Minimum Capital Requirements	236,321	221,066
Minimum to be Well Capitalized Under Action Provisions	$ 295,402	$ 276,333
Total Risk-based Capital, Ratio
Actual	0.1120	0.1116
Minimum Capital Requirements	0.0800	0.0800
Minimum to be Well Capitalized Under Action Provisions	0.1000	0.1000
Tier 1 Capital, Amount
Actual	$ 304,553	$ 284,992
Minimum Capital Requirements	177,241	165,800
Minimum to be Well Capitalized Under Action Provisions	$ 236,321	$ 221,066
Tier 1 Capital, Ratio
Actual	0.1031	0.1031
Minimum Capital Requirements	0.0600	0.0600
Minimum to be Well Capitalized Under Action Provisions	0.0800	0.0800
Tier 1 Leverage Capital, Amount
Actual	$ 304,553	$ 284,992
Minimum Capital Requirements	121,821	121,884
Minimum to be Well Capitalized Under Action Provisions	$ 152,277	$ 152,355
Tier 1 Leverage Capital, Ratio
Actual	0.0894	0.0935
Minimum Capital Requirements	0.0400	0.0400
Minimum to be Well Capitalized Under Action Provisions	0.0500	0.0500
Common Equity Tier One Capital, Amount
Actual	$ 304,553	$ 284,992
Minimum Capital Requirements	132,931	124,350
Minimum to be Well Capitalized Under Action Provisions	$ 192,011	$ 179,616
Common Equity Tier One Capital, Ratio
Actual	0.1031	0.1031
Minimum Capital Requirements	4.50%	4.50%
Minimum to be Well Capitalized Under Action Provisions	6.50%	6.50%