Derivative Instruments and Hedging Activities (Schedule of Impact of Derivatives on Balance Sheet) (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Designated as hedging instruments
Derivative [Line Items]
Estimated fair value of derivative assets	$ 6,114	$ 0
Estimated fair value of derivative liabilities	12,252	16,379
Derivatives not designated as hedging instruments
Derivative [Line Items]
Estimated fair value of derivative assets	155,049	89,815
Estimated fair value of derivative liabilities	376,509	326,301
Derivatives not designated as hedging instruments \| Interest rate contracts
Derivative [Line Items]
Notional Amount	11,343,848	10,276,754
Estimated fair value of derivative assets	153,052	89,310
Estimated fair value of derivative liabilities	374,865	322,329
Derivatives not designated as hedging instruments \| Mortgage derivatives \| Mortgage derivatives - interest rate lock commitments
Derivative [Line Items]
Notional Amount	57,643	50,218
Estimated fair value of derivative assets	632	350
Estimated fair value of derivative liabilities	0	0
Derivatives not designated as hedging instruments \| Mortgage derivatives \| Mortgage derivatives - forward commitments to sell fixed-rate mortgage loans
Derivative [Line Items]
Notional Amount	86,234	76,500
Estimated fair value of derivative assets	728	155
Estimated fair value of derivative liabilities	0	0
Derivatives not designated as hedging instruments \| Risk participation agreements
Derivative [Line Items]
Notional Amount	708,259	635,891
Estimated fair value of derivative assets	0	0
Estimated fair value of derivative liabilities	1	3
Derivatives not designated as hedging instruments \| Foreign exchange contracts
Derivative [Line Items]
Notional Amount	41,420	20,439
Estimated fair value of derivative assets	637	0
Estimated fair value of derivative liabilities	0	516
Derivatives not designated as hedging instruments \| Visa derivative
Derivative [Line Items]
Notional Amount	0	0
Estimated fair value of derivative assets	0	0
Estimated fair value of derivative liabilities	1,643	3,453
Cash flow hedges
Derivative [Line Items]
Estimated fair value of derivative assets	6,114	0
Estimated fair value of derivative liabilities	0	8,286
Cash flow hedges \| Interest rate contracts
Derivative [Line Items]
Notional Amount	5,750,000	5,250,000
Estimated fair value of derivative assets	6,114	0
Estimated fair value of derivative liabilities	0	8,286
Fair value hedges
Derivative [Line Items]
Estimated fair value of derivative assets	0	0
Estimated fair value of derivative liabilities	12,252	8,093
Fair value hedges \| Interest rate contracts
Derivative [Line Items]
Notional Amount	2,773,504	2,230,232
Estimated fair value of derivative assets	0	0
Estimated fair value of derivative liabilities	$ 12,252	$ 8,093

Fair value, after offset of derivative liability, of financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, before offset against an obligation to return collateral under a master netting arrangement. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 55
-Paragraph 9
-SubTopic 20
-Topic 210
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483444/210-20-55-9

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (c)
-SubTopic 20
-Topic 210
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483466/210-20-50-3

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 55
-Paragraph 10
-SubTopic 20
-Topic 210
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483444/210-20-55-10

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480434/815-10-50-8

+ Details

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

- Definition

Fair value, after offset of derivative asset, of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected not to be and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

+ Details

Name:	us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480237/815-40-50-5

+ Details

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

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480434/815-10-50-1B

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1A
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480434/815-10-50-1A

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=syn_MortgageDerivativeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_InterestRateLockCommitmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=syn_ForwardCommitmentsToSellFixedRateMortgageLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=syn_RiskParticipationAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=syn_VisaDerivativeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_FairValueHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: