Investment Securities Available for Sale - Schedule Of Unrealized Loss On Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, fair value	$ 6,596,065	$ 1,379,662
Less than 12 months, gross unrealized losses	(96,227)	(16,669)
12 months or longer, fair value	1,523,383	0
12 months or longer, gross unrealized losses	(43,377)	0
Total, fair value	8,119,448	1,379,662
Total, gross unrealized losses	(139,604)	(16,669)
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, fair value	49,648
Less than 12 months, gross unrealized losses	(379)
12 months or longer, fair value	47,590
12 months or longer, gross unrealized losses	(2,248)
Total, fair value	97,238
Total, gross unrealized losses	(2,627)
U.S. Government agency securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, fair value	21,760
Less than 12 months, gross unrealized losses	(387)
12 months or longer, fair value	0
12 months or longer, gross unrealized losses	0
Total, fair value	21,760
Total, gross unrealized losses	(387)
Mortgage-backed securities issued by U.S. Government agencies
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, fair value	461,078	566,896
Less than 12 months, gross unrealized losses	(5,858)	(5,925)
12 months or longer, fair value	244,264	0
12 months or longer, gross unrealized losses	(5,606)	0
Total, fair value	705,342	566,896
Total, gross unrealized losses	(11,464)	(5,925)
Mortgage-backed securities issued by U.S. Government sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, fair value	5,729,476
Less than 12 months, gross unrealized losses	(82,671)
12 months or longer, fair value	643,758
12 months or longer, gross unrealized losses	(19,409)
Total, fair value	6,373,234
Total, gross unrealized losses	(102,080)
Collateralized mortgage obligations issued by U.S. Government agencies or sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, fair value	187,431	803,429
Less than 12 months, gross unrealized losses	(3,981)	(10,576)
12 months or longer, fair value	504,238	0
12 months or longer, gross unrealized losses	(12,745)	0
Total, fair value	691,669	803,429
Total, gross unrealized losses	(16,726)	(10,576)
Commercial mortgage-backed securities issued by U.S. Government agencies or sponsored enterprises
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, fair value	146,672
Less than 12 months, gross unrealized losses	(2,951)
12 months or longer, fair value	83,533
12 months or longer, gross unrealized losses	(3,369)
Total, fair value	230,205
Total, gross unrealized losses	$ (6,320)
Corporate debt securities and other debt securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, fair value		9,337
Less than 12 months, gross unrealized losses		(168)
12 months or longer, fair value		0
12 months or longer, gross unrealized losses		0
Total, fair value		9,337
Total, gross unrealized losses		$ (168)

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=124269663&loc=SL82922890-210455

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27337-111563

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=124269663&loc=SL82922890-210455

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=124269663&loc=SL82922888-210455

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456

+ Details

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

- Definition

Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=syn_CommercialmortgagebackedsecuritiesissuedbyU.S.GovernmentsponsoredagenciesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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