Loans And Allowance For Loan Losses (Schedule Of Impaired Loans) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	$ 11,022	$ 28,751
Unpaid Principal Balance, With no related allowance recorded	14,259	40,794
Related Allowance, With no related allowance recorded	0	0
Average Recorded Investment, With no related allowance recorded	32,314	26,433
Interest Income Recognized, With no related allowance recorded	0	0
Recorded Investment, With allowance recorded	189,264	220,745
Unpaid Principal Balance, With allowance recorded	189,498	223,512
Related Allowance, With allowance recorded	14,908	18,111
Average Recorded Investment, With allowance recorded	190,695	248,082
Interest Income Recognized, With allowance recorded	6,961	8,196
Recorded Investment	200,286	249,496
Unpaid Principal Balance	203,757	264,306
Related Allowance	14,908	18,111
Average Recorded Investment in Impaired Loans	223,009	274,515	$ 358,300
Interest Income Recognized	6,961	8,196
Total commercial real estate
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	56	3,490
Unpaid Principal Balance, With no related allowance recorded	1,740	10,975
Related Allowance, With no related allowance recorded	0	0
Average Recorded Investment, With no related allowance recorded	2,262	9,803
Interest Income Recognized, With no related allowance recorded	0	0
Recorded Investment, With allowance recorded	56,840	87,920
Unpaid Principal Balance, With allowance recorded	56,896	87,926
Related Allowance, With allowance recorded	4,240	7,916
Average Recorded Investment, With allowance recorded	68,344	108,091
Interest Income Recognized, With allowance recorded	2,473	3,286
Recorded Investment	56,896	91,410
Unpaid Principal Balance	58,636	98,901
Related Allowance	4,240	7,916
Average Recorded Investment in Impaired Loans	70,606	117,894
Interest Income Recognized	2,473	3,286
Total commercial real estate \| Investment properties
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	0	748
Unpaid Principal Balance, With no related allowance recorded	0	793
Related Allowance, With no related allowance recorded	0	0
Average Recorded Investment, With no related allowance recorded	123	2,013
Interest Income Recognized, With no related allowance recorded	0	0
Recorded Investment, With allowance recorded	23,364	31,489
Unpaid Principal Balance, With allowance recorded	23,364	31,489
Related Allowance, With allowance recorded	1,100	2,044
Average Recorded Investment, With allowance recorded	28,749	42,659
Interest Income Recognized, With allowance recorded	1,144	1,436
Recorded Investment	23,364	32,237
Unpaid Principal Balance	23,364	32,282
Related Allowance	1,100	2,044
Average Recorded Investment in Impaired Loans	28,872	44,672
Interest Income Recognized	1,144	1,436
Total commercial real estate \| 1-4 family properties
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	0	643
Unpaid Principal Balance, With no related allowance recorded	0	2,939
Related Allowance, With no related allowance recorded	0	0
Average Recorded Investment, With no related allowance recorded	323	1,021
Interest Income Recognized, With no related allowance recorded	0	0
Recorded Investment, With allowance recorded	15,056	23,642
Unpaid Principal Balance, With allowance recorded	15,056	23,649
Related Allowance, With allowance recorded	504	769
Average Recorded Investment, With allowance recorded	16,257	39,864
Interest Income Recognized, With allowance recorded	925	855
Recorded Investment	15,056	24,285
Unpaid Principal Balance	15,056	26,588
Related Allowance	504	769
Average Recorded Investment in Impaired Loans	16,580	40,885
Interest Income Recognized	925	855
Total commercial real estate \| Land and development
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	56	2,099
Unpaid Principal Balance, With no related allowance recorded	1,740	7,243
Related Allowance, With no related allowance recorded	0	0
Average Recorded Investment, With no related allowance recorded	1,816	6,769
Interest Income Recognized, With no related allowance recorded	0	0
Recorded Investment, With allowance recorded	18,420	32,789
Unpaid Principal Balance, With allowance recorded	18,476	32,788
Related Allowance, With allowance recorded	2,636	5,103
Average Recorded Investment, With allowance recorded	23,338	25,568
Interest Income Recognized, With allowance recorded	404	995
Recorded Investment	18,476	34,888
Unpaid Principal Balance	20,216	40,031
Related Allowance	2,636	5,103
Average Recorded Investment in Impaired Loans	25,154	32,337
Interest Income Recognized	404	995
Total commercial and industrial
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	8,220	23,466
Unpaid Principal Balance, With no related allowance recorded	9,576	27,954
Related Allowance, With no related allowance recorded	0	0
Average Recorded Investment, With no related allowance recorded	28,351	14,715
Interest Income Recognized, With no related allowance recorded	0	0
Recorded Investment, With allowance recorded	103,114	97,094
Unpaid Principal Balance, With allowance recorded	103,292	99,855
Related Allowance, With allowance recorded	9,515	8,384
Average Recorded Investment, With allowance recorded	90,966	104,268
Interest Income Recognized, With allowance recorded	2,992	3,161
Recorded Investment	111,334	120,560
Unpaid Principal Balance	112,868	127,809
Related Allowance	9,515	8,384
Average Recorded Investment in Impaired Loans	119,317	118,983
Interest Income Recognized	2,992	3,161
Total commercial and industrial \| Commercial, financial and agricultural
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	8,220	17,958
Unpaid Principal Balance, With no related allowance recorded	9,576	20,577
Related Allowance, With no related allowance recorded	0	0
Average Recorded Investment, With no related allowance recorded	21,686	6,321
Interest Income Recognized, With no related allowance recorded	0	0
Recorded Investment, With allowance recorded	65,715	43,386
Unpaid Principal Balance, With allowance recorded	65,851	45,913
Related Allowance, With allowance recorded	7,406	5,687
Average Recorded Investment, With allowance recorded	50,468	51,968
Interest Income Recognized, With allowance recorded	1,610	1,215
Recorded Investment	73,935	61,344
Unpaid Principal Balance	75,427	66,490
Related Allowance	7,406	5,687
Average Recorded Investment in Impaired Loans	72,154	58,289
Interest Income Recognized	1,610	1,215
Total commercial and industrial \| Owner-occupied
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	0	5,508
Unpaid Principal Balance, With no related allowance recorded	0	7,377
Related Allowance, With no related allowance recorded	0	0
Average Recorded Investment, With no related allowance recorded	6,665	8,394
Interest Income Recognized, With no related allowance recorded	0	0
Recorded Investment, With allowance recorded	37,399	53,708
Unpaid Principal Balance, With allowance recorded	37,441	53,942
Related Allowance, With allowance recorded	2,109	2,697
Average Recorded Investment, With allowance recorded	40,498	52,300
Interest Income Recognized, With allowance recorded	1,382	1,946
Recorded Investment	37,399	59,216
Unpaid Principal Balance	37,441	61,319
Related Allowance	2,109	2,697
Average Recorded Investment in Impaired Loans	47,163	60,694
Interest Income Recognized	1,382	1,946
Total consumer
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	2,746	1,795
Unpaid Principal Balance, With no related allowance recorded	2,943	1,865
Related Allowance, With no related allowance recorded	0	0
Average Recorded Investment, With no related allowance recorded	1,701	1,915
Interest Income Recognized, With no related allowance recorded	0	0
Recorded Investment, With allowance recorded	29,310	35,731
Unpaid Principal Balance, With allowance recorded	29,310	35,731
Related Allowance, With allowance recorded	1,153	1,811
Average Recorded Investment, With allowance recorded	31,385	35,723
Interest Income Recognized, With allowance recorded	1,496	1,749
Recorded Investment	32,056	37,526
Unpaid Principal Balance	32,253	37,596
Related Allowance	1,153	1,811
Average Recorded Investment in Impaired Loans	33,086	37,638
Interest Income Recognized	1,496	1,749
Total consumer \| Home equity lines
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	2,746	1,051
Unpaid Principal Balance, With no related allowance recorded	2,943	1,051
Related Allowance, With no related allowance recorded	0	0
Average Recorded Investment, With no related allowance recorded	1,205	1,045
Interest Income Recognized, With no related allowance recorded	0	0
Recorded Investment, With allowance recorded	5,096	9,638
Unpaid Principal Balance, With allowance recorded	5,096	9,638
Related Allowance, With allowance recorded	114	971
Average Recorded Investment, With allowance recorded	7,476	9,668
Interest Income Recognized, With allowance recorded	334	432
Recorded Investment	7,842	10,689
Unpaid Principal Balance	8,039	10,689
Related Allowance	114	971
Average Recorded Investment in Impaired Loans	8,681	10,713
Interest Income Recognized	334	432
Total consumer \| Consumer mortgages
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	0	744
Unpaid Principal Balance, With no related allowance recorded	0	814
Related Allowance, With no related allowance recorded	0	0
Average Recorded Investment, With no related allowance recorded	496	870
Interest Income Recognized, With no related allowance recorded	0	0
Recorded Investment, With allowance recorded	18,668	20,953
Unpaid Principal Balance, With allowance recorded	18,668	20,953
Related Allowance, With allowance recorded	569	673
Average Recorded Investment, With allowance recorded	19,144	20,993
Interest Income Recognized, With allowance recorded	896	1,014
Recorded Investment	18,668	21,697
Unpaid Principal Balance	18,668	21,767
Related Allowance	569	673
Average Recorded Investment in Impaired Loans	19,640	21,863
Interest Income Recognized	896	1,014
Total consumer \| Credit cards
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	0	0
Unpaid Principal Balance, With no related allowance recorded	0	0
Related Allowance, With no related allowance recorded	0	0
Average Recorded Investment, With no related allowance recorded	0	0
Interest Income Recognized, With no related allowance recorded	0	0
Recorded Investment, With allowance recorded	0	0
Unpaid Principal Balance, With allowance recorded	0	0
Related Allowance, With allowance recorded	0	0
Average Recorded Investment, With allowance recorded	0	0
Interest Income Recognized, With allowance recorded	0	0
Recorded Investment	0	0
Unpaid Principal Balance	0	0
Related Allowance	0	0
Average Recorded Investment in Impaired Loans	0	0
Interest Income Recognized	0	0
Total consumer \| Other consumer loans
Financing Receivable, Impaired [Line Items]
Recorded Investment, With no related allowance recorded	0	0
Unpaid Principal Balance, With no related allowance recorded	0	0
Related Allowance, With no related allowance recorded	0	0
Average Recorded Investment, With no related allowance recorded	0	0
Interest Income Recognized, With no related allowance recorded	0	0
Recorded Investment, With allowance recorded	5,546	5,140
Unpaid Principal Balance, With allowance recorded	5,546	5,140
Related Allowance, With allowance recorded	470	167
Average Recorded Investment, With allowance recorded	4,765	5,062
Interest Income Recognized, With allowance recorded	266	303
Recorded Investment	5,546	5,140
Unpaid Principal Balance	5,546	5,140
Related Allowance	470	167
Average Recorded Investment in Impaired Loans	4,765	5,062
Interest Income Recognized	$ 266	$ 303

Impaired Financing Receivable, with No Related Allowance, Related Allowance

No definition available.

Name:	syn_ImpairedFinancingReceivablewithNoRelatedAllowanceRelatedAllowance
Namespace Prefix:	syn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Impaired Financing Receivable, with Related Allowance, Related Allowance

+ References

No definition available.

+ Details

Name:	syn_ImpairedFinancingReceivablewithRelatedAllowanceRelatedAllowance
Namespace Prefix:	syn_
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

No definition available.

+ Details

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

- Definition

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5258-111524

+ Details

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

- Definition

Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

+ Details

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

- Definition

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953550-111524

+ Details

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

- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

+ Details

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

- Definition

Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of average investment of impaired financing receivables for which there is no related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5258-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

+ Details

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

- Definition

Amount of interest income, recognized on accrual basis, on financing receivables that are recognized as impaired, for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of average investment of impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of interest income, recognized on accrual basis, on financing receivables, for which there is a related allowance for credit losses and are recognized as impaired.

+ References

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Definition

Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- 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_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=syn_LandAcquisitionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- 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_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=syn_OwnerOccupiedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CreditCardReceivablesMember
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: