Long-Term Debt - Summary of long term debt information (Details)	6 Months Ended
Jun. 30, 2020 USD ($)
Paycheck Protection Program Loan [Member]
Debt Instrument
Percentage of fixed interest rate payable	1.00%
Line of credit payable to a financial institution; Interest rate option of bank prime or Libor plus 1.65% (1.579% as of June 30, 2020) (B) (C )
Debt Instrument
Spread on variable rate	1.65%
Percentage of fixed interest rate payable	1.579%
Term loan payable to a financial institution; Interest rate option of bank prime or Libor plus 1.4% (1.573% as of June 30, 2020), monthly principal payments of $21,833 through 2021 with a balloon payment of $786,000 due December 1, 2021 (C)
Debt Instrument
Percentage of fixed interest rate payable	1.573%
Monthly principal payments	$ 21,833
Balloon payment due December 1, 2021	$ 786,000
Term loan payable to a financial institution; Interest rate option of bank prime or Libor plus 1.4% (2.981130% as of June 30, 2020), monthly principal payments of $23,810 through December 1, 2021 (C)
Debt Instrument
Percentage of fixed interest rate payable	2.98113%
Monthly principal payments	$ 23,810
Equipment note obligations; Interest rate fixed for term of each funding based upon the Lender's lease pricing at time of funding. (Interest rate/factor 1.8259% - 1.835015% as of June 30, 2020) (D) \| Minimum
Debt Instrument
Percentage of floating interest rate payable	1.8259%
Equipment note obligations; Interest rate fixed for term of each funding based upon the Lender's lease pricing at time of funding. (Interest rate/factor 1.8259% - 1.835015% as of June 30, 2020) (D) \| Maximum
Debt Instrument
Percentage of floating interest rate payable	1.83501%
Equipment financing lease obligations; Interest rate fixed for term of each funding based upon the Lender's lease pricing at time of funding. (Interest rate/factor 1.822758% - 1.869304% at time of funding) (E) \| Minimum
Debt Instrument
Percentage of floating interest rate payable	1.82276%
Equipment financing lease obligations; Interest rate fixed for term of each funding based upon the Lender's lease pricing at time of funding. (Interest rate/factor 1.822758% - 1.869304% at time of funding) (E) \| Maximum
Debt Instrument
Percentage of floating interest rate payable	1.8693%
LIBOR \| Line of credit payable to a financial institution; Interest rate option of bank prime or Libor plus 1.65% (1.579% as of June 30, 2020) (B) (C )
Debt Instrument
Spread on variable rate	1.65%
LIBOR \| Term loan payable to a financial institution; Interest rate option of bank prime or Libor plus 1.4% (1.573% as of June 30, 2020), monthly principal payments of $21,833 through 2021 with a balloon payment of $786,000 due December 1, 2021 (C)
Debt Instrument
Spread on variable rate	1.40%
LIBOR \| Term loan payable to a financial institution; Interest rate option of bank prime or Libor plus 1.4% (2.981130% as of June 30, 2020), monthly principal payments of $23,810 through December 1, 2021 (C)
Debt Instrument
Spread on variable rate	1.40%

This element represents final payment including both interest and principal payment.

No definition available.

Name:	svt_DebtInstrumentFinalPayment
Namespace Prefix:	svt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6036836-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateEffectivePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of the required periodic payments including both interest and principal payments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

+ Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=svt_PaycheckProtectionProgramLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_LoansPayableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=svt_TermLoanPayableToFinancialInstitutionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=svt_LineOfCreditLeaseForEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=svt_LineOfCredit2000000Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: