LOANS AND ALLOWANCE FOR LOAN LOSSES (Schedule of Activity for Loan and Lease Losses By Class of Loan) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 10,261	$ 12,283
Provision (recovery)	3,450	200
Charge offs	(1,099)	(689)
Recoveries (adjustment)	110	80
Ending balance	12,722	11,874
Commercial Real Estate Portfolio Segment [Member] \| Commercial Real Estate Owner Occupied
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	810	802
Provision (recovery)	253	11
Recoveries (adjustment)	5	3
Ending balance	1,068	816
Commercial Real Estate Portfolio Segment [Member] \| Commercial Real Estate Non-owner Occupied
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	1,720	1,669
Provision (recovery)	971	624
Charge offs		(462)
Recoveries (adjustment)	2
Ending balance	2,693	1,831
Commercial Real Estate Portfolio Segment [Member] \| Construction Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	683
Ending balance	376
Residential Portfolio Segment [Member] \| 1-4 Family Residential
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	1,266	1,106
Provision (recovery)	1,387	56
Charge offs	(245)
Recoveries (adjustment)	31	8
Ending balance	2,439	1,170
Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	5,418	7,097
Provision (recovery)	1,172	(887)
Charge offs	(822)	(167)
Recoveries (adjustment)	65	63
Ending balance	5,833	6,106
Commercial Portfolio Segment [Member] \| Construction Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	683	821
Provision (recovery)	(307)	99
Ending balance	376	920
Consumer Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	190	224
Provision (recovery)	74	83
Charge offs	(32)	(60)
Recoveries (adjustment)	7	6
Ending balance	239	253
Unallocated
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	174	564
Provision (recovery)	(100)	214
Ending balance	74	$ 778
Receivables Acquired with Deteriorated Credit Quality [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Provision (recovery)	350
Receivables Acquired with Deteriorated Credit Quality [Member] \| Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Provision (recovery)	350
Receivables With Deteriorated Credit Quality [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Provision (recovery)	3,100
Receivables With Deteriorated Credit Quality [Member] \| Commercial Real Estate Portfolio Segment [Member] \| Commercial Real Estate Owner Occupied
Allowance for Loan and Lease Losses [Roll Forward]
Provision (recovery)	253
Receivables With Deteriorated Credit Quality [Member] \| Commercial Real Estate Portfolio Segment [Member] \| Commercial Real Estate Non-owner Occupied
Allowance for Loan and Lease Losses [Roll Forward]
Provision (recovery)	971
Receivables With Deteriorated Credit Quality [Member] \| Residential Portfolio Segment [Member] \| 1-4 Family Residential
Allowance for Loan and Lease Losses [Roll Forward]
Provision (recovery)	1,387
Receivables With Deteriorated Credit Quality [Member] \| Commercial Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Provision (recovery)	822
Receivables With Deteriorated Credit Quality [Member] \| Commercial Portfolio Segment [Member] \| Construction Loans [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Provision (recovery)	(307)
Receivables With Deteriorated Credit Quality [Member] \| Consumer Portfolio Segment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Provision (recovery)	74
Receivables With Deteriorated Credit Quality [Member] \| Unallocated
Allowance for Loan and Lease Losses [Roll Forward]
Provision (recovery)	$ (100)

Allowance for loan and lease loss recovered (written off adjustment) of bad debts.

No definition available.

Name:	sona_AllowanceForLoanAndLeaseLossRecoveredWrittenOffAdjustmentOfBadDebts
Namespace Prefix:	sona_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of other addition (reduction) in allowance reserve for credit loss.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5212-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=118939070&loc=d3e15032-111544

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

+ Details

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

- Details

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

- Details

Name:	us-gaap_ClassOfFinancingReceivableTypeOfBorrowerAxis=sona_CommercialRealEstateOwnerOccupiedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ClassOfFinancingReceivableTypeOfBorrowerAxis=sona_CommercialRealEstateNonOwnerOccupiedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
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_FinancingReceivablePortfolioSegmentAxis=us-gaap_UnallocatedFinancingReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditLossStatusAxis=us-gaap_FinancialAssetAcquiredWithCreditDeteriorationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditLossStatusAxis=sona_ReceivablesWithDeterioratedCreditQualityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: