Loans and Allowance for Loan Losses (Financial Effect of Loan Modifications Made To Borrowers Experiencing Financial Difficulty) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2023	Jun. 30, 2023
Total commercial and industrial \| Commercial, financial and agricultural
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Weighted Average Interest Rate Reduction	1.10%	1.20%
Weighted Average Term Extension (in months)	41 months	28 months
Total commercial and industrial \| Commercial, financial and agricultural \| Principal Forgiveness and Term Extensions
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Principal Forgiveness and Term Extensions	$ 1,200	$ 1,200
Total commercial and industrial \| Owner-occupied
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Weighted Average Interest Rate Reduction	0.00%	1.70%
Weighted Average Term Extension (in months)	17 months	9 months
Total commercial and industrial \| Owner-occupied \| Principal Forgiveness and Term Extensions
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Principal Forgiveness and Term Extensions	$ 0	$ 0
Total commercial real estate \| Investment properties
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Weighted Average Interest Rate Reduction	0.00%	0.00%
Weighted Average Term Extension (in months)	30 months	30 months
Total commercial real estate \| Investment properties \| Principal Forgiveness and Term Extensions
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Principal Forgiveness and Term Extensions	$ 0	$ 0
Total commercial real estate \| 1-4 family properties
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Weighted Average Interest Rate Reduction	0.30%	0.30%
Weighted Average Term Extension (in months)	12 months	12 months
Total commercial real estate \| 1-4 family properties \| Principal Forgiveness and Term Extensions
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Principal Forgiveness and Term Extensions	$ 0	$ 0
Total consumer \| Mortgage Reporting Unit
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Weighted Average Interest Rate Reduction	1.90%	1.60%
Weighted Average Term Extension (in months)	0 months	0 months
Total consumer \| Mortgage Reporting Unit \| Principal Forgiveness and Term Extensions
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Principal Forgiveness and Term Extensions	$ 0	$ 0
Total consumer \| Home equity
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Weighted Average Interest Rate Reduction	0.40%	0.50%
Weighted Average Term Extension (in months)	250 months	262 months
Total consumer \| Home equity \| Principal Forgiveness and Term Extensions
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Principal Forgiveness and Term Extensions	$ 0	$ 0
Total consumer \| Other consumer loans
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Weighted Average Interest Rate Reduction	2.70%	3.10%
Weighted Average Term Extension (in months)	61 months	64 months
Total consumer \| Other consumer loans \| Principal Forgiveness and Term Extensions
Troubled Debt Restructuring, Debtor, Current Period [Line Items]
Principal Forgiveness and Term Extensions	$ 0	$ 0

Amount of increase (decrease) in amortized cost excluding accrued interest, of financing receivable modified for debtor experiencing financial difficulty.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-42

+ Details

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

- Definition

Percentage of decrease in weighted average contractual interest rate of financing receivable modified for debtor experiencing financial difficulty.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481933/310-10-55-12A

+ Details

Name:	us-gaap_FinancingReceivableModifiedWeightedAverageInterestRateDecreaseFromModification
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Period of increase in weighted average maturity of financing receivable modified for debtor experiencing financial difficulty, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModifiedWeightedAverageTermIncreaseFromModification
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

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.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=syn_CommercialAndIndustrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=syn_CommercialFinancialandAgriculturalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ExtendedMaturityAndPrincipalForgivenessMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=syn_OwnerOccupiedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=syn_InvestmentPropertiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=syn_OneToFourFamilyPropertiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=syn_ConsumerMortgagesReportingUnitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=syn_OtherConsumerLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: