Name:	fgbi_DebtSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAsPercentageOfAllIndividualSecuritiesAmortizedCostBasis
Namespace Prefix:	fgbi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Refers to accumulated unrealized loss on investments in debt securities in a continuous loss position as percentage of investment securities portfolio.

No definition available.

Name:	fgbi_DebtSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAsPercentageOfInvestmentSecuritiesPortfolio
Namespace Prefix:	fgbi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Amount of accumulated unrealized loss on investments in debt securities in a continuous loss position.

No definition available.

Name:	fgbi_SecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
Namespace Prefix:	fgbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

This item represents the amortized cost of securities that have been in a continuous unrealized loss position for twelve months or longer.

No definition available.

Name:	fgbi_SecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerAmortizedCostBasis
Namespace Prefix:	fgbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Number of debt securities that have been in a continuous debt position of unrealized loss position for greater than or equal to a year for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

No definition available.

Name:	fgbi_SecuritiesInUnrealizedLossPositionsMoreThanTwelveMonthsQualitativeDisclosureNumberOfPositions
Namespace Prefix:	fgbi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Number of debt securities in a continuous debt positions of unrealized loss for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

No definition available.

Name:	fgbi_SecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
Namespace Prefix:	fgbi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Amount of investment in debt security, measured at amortized cost, sold.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27405-111563

Name:	us-gaap_DebtSecuritiesHeldToMaturitySoldAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Amount of gain (loss) on sale of investment in debt security measured at amortized cost (held-to-maturity), including other-than-temporary impairment (OTTI).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27405-111563

Name:	us-gaap_HeldToMaturitySecuritiesSoldSecurityRealizedGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

Name:	us-gaap_InvestmentsDebtAndEquitySecuritiesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which no such other than temporary impairment (OTTI) was previously recognized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 8B
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=SL6284393-111563

Name:	us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsNoPreviousImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Sum of the carrying amounts as of the date of the latest financial statement presented of securities which are owned but transferred to another party to serve as collateral to partially or fully secure a debt obligation, repurchase agreement or other current or potential obligation, and for which the transferee is not permitted to sell or re-pledge them to an unrelated party.

Name:	us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Fair value of financial instrument held by the entity for its own account (proprietary securities) for trading or investment purposes that are carried at fair value and pledged to counterparties as collateral for financing transactions.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728