Long-Term Debt - Summary of long term debt (Parentheticals) (Details) $ in Thousands	3 Months Ended
Mar. 31, 2019 USD ($)
Term loan payable to a financial institution; Interest rate option of bank prime or Libor plus 1.4% (3.909% as of March 31, 2019), monthly principal payments of $21,833 through 2021 with a balloon payment of $786,000 due December 1, 2021
Debt Instrument [Line Items]
Description of rate basis	Libor
Percentage of floating interest rate payable	1.40%
Percentage of fixed interest rate payable	3.909%
Frequency of principal payments	Monthly
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% (3.909% as of March 31, 2019), monthly principal payments of $23,810 through December 1, 2021
Debt Instrument [Line Items]
Description of rate basis	Libor
Percentage of floating interest rate payable	1.40%
Percentage of fixed interest rate payable	3.909%
Frequency of principal payments	Monthly
Monthly principal payments	$ 23,810
Lease line of credit for equipment; 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), monthly principle payments of $12,675 through April 10, 2023
Debt Instrument [Line Items]
Description of rate basis	Interest rate/factor
Frequency of principal payments	Monthly
Monthly principal payments	$ 12,675
Lease line of credit for equipment; 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), monthly principle payments of $12,675 through April 10, 2023 \| Minimum
Debt Instrument [Line Items]
Percentage of floating interest rate payable	1.82276%
Lease line of credit for equipment; 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), monthly principle payments of $12,675 through April 10, 2023 \| Maximum
Debt Instrument [Line Items]
Percentage of floating interest rate payable	1.8693%

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

No definition available.

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

- Definition

The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Description of the frequency of periodic payments (monthly, quarterly, annual).

+ 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

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-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

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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

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-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

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-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
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-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	na
Period Type:	instant

- Definition

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

+ References

+ Details

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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

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Period Type:	duration

- Definition

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

+ References

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