Capital Requirements - Actual Capital Amounts and Ratios (Details) $ in Thousands	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Jun. 30, 2020
Tier 1 Leverage Capital, Amount
Actual	$ 10,000,000
Tier 1 Leverage Capital, Ratio
Actual			0.08
First Guaranty Bank
Total Risk-based Capital, Amount
Actual	308,510	$ 268,002
Minimum Capital Requirements	221,066	191,069
Minimum to be Well Capitalized Under Action Provisions	$ 276,333	$ 238,837
Total Risk-based Capital, Ratio
Actual	0.1116	0.1122
Minimum Capital Requirements	0.0800	0.0800
Minimum to be Well Capitalized Under Action Provisions	0.1000	0.1000
Tier 1 Capital, Amount
Actual	$ 284,992	$ 243,973
Minimum Capital Requirements	165,800	143,302
Minimum to be Well Capitalized Under Action Provisions	$ 221,066	$ 191,069
Tier 1 Capital, Ratio
Actual	0.1031	0.1022
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	$ 284,992	$ 243,973
Minimum Capital Requirements	121,884	112,018
Minimum to be Well Capitalized Under Action Provisions	$ 152,355	$ 140,023
Tier 1 Leverage Capital, Ratio
Actual	0.0935	0.0871
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	$ 284,992	$ 243,973
Minimum Capital Requirements	124,350	107,476
Minimum to be Well Capitalized Under Action Provisions	$ 179,616	$ 155,244
Common Equity Tier One Capital, Ratio
Actual	0.1031	0.1022
Minimum Capital Requirements	4.50%	4.50%
Minimum to be Well Capitalized Under Action Provisions	6.50%	6.50%