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Deposits - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Interest-Bearing Deposit Liabilities [Abstract]
Aggregate amount of time deposits	$ 3,720	$ 3,550

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- Definition

Time Deposits, $250,000 or More

+ References

No definition available.

+ Details

Name:	syn_TimeDeposits250000orMore
Namespace Prefix:	syn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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- References

No definition available.

+ Details

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