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

No definition available.

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

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

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

- 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/presentationRef
-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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

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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

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

- Definition

Amount, after accumulated amortization, of debt discount.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

+ Details

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

- Definition

The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used.

+ References

No definition available.

+ Details

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

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

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

- Definition

Debt Instrument, Basis Spread on Variable Rate, Reduction

+ References

No definition available.

+ Details

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Namespace Prefix:	vsm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument, Covenant, Leverage Ratio

+ References

No definition available.

+ Details

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Namespace Prefix:	vsm_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt Instrument, Covenant, Leverage Ratio, Maximum

+ References

No definition available.

+ Details

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Namespace Prefix:	vsm_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt Instrument, Prepayment Amount From Excess Cash Flows, Percentage

+ References

No definition available.

+ Details

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Namespace Prefix:	vsm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument, Variable Rate Floor

+ References

No definition available.

+ Details

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Namespace Prefix:	vsm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line of Credit Facility, Fronting Fee Percentage

+ References

No definition available.

+ Details

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Namespace Prefix:	vsm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Term Facility Percent Original Principal Annual Amortization

+ References

No definition available.

+ Details

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Namespace Prefix:	vsm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=vsm_PriorFourQuarterPeriodMember
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Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Balance Type:
Period Type:

- Details

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Namespace Prefix:
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Period Type:

- Details

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

- Details

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Balance Type:
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- Details

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Balance Type:
Period Type:

- Details

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Data Type:	na
Balance Type:
Period Type:

- Details

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Debt (Narrative) (Details)		12 Months Ended
Debt (Narrative) (Details)	Oct. 10, 2017	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
Debt Instrument [Line Items]
Original issue discount		$ 2,500,000	$ 2,800,000
Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 200,000,000
Commitment fee percentage		0.375%
Fronting fee percentage		0.125%
Maximum net leverage ratio		3.25
Remaining borrowing capacity		$ 200,000,000
Line of Credit \| Revolving Credit Facility \| Prior Four Quarter Period
Debt Instrument [Line Items]
Net leverage ratio		1.00
Line of Credit \| Revolving Credit Facility \| Prior Four Quarter Period After Delivery of Financial Statements
Debt Instrument [Line Items]
Net leverage ratio		1.00
Line of Credit \| Revolving Credit Facility \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate		2.00%
Basis spread on variable rate, reduction		0.25%
Line of Credit \| Revolving Credit Facility \| LIBOR \| Prior Four Quarter Period After Delivery of Financial Statements
Debt Instrument [Line Items]
Basis spread on variable rate, reduction		0.25%
Line of Credit \| Revolving Credit Facility \| Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate		1.00%
Term Loan Facility
Debt Instrument [Line Items]
Debt instrument, face amount		$ 575,000,000
Effective interest rate (as percent)		3.83%
Term facility, percent original principal annual amortization		1.00%
Term Loan Facility \| LIBOR
Debt Instrument [Line Items]
Minimum floor (as percent)		0.75%
Basis spread on variable rate		2.50%
Term Loan Facility \| Base Rate
Debt Instrument [Line Items]
Minimum floor (as percent)		1.75%
Basis spread on variable rate		1.50%
Term Loan Facility \| Minimum
Debt Instrument [Line Items]
Prepayment amount of excess cash flows, percentage		0.00%
Term Loan Facility \| Maximum
Debt Instrument [Line Items]
Prepayment amount of excess cash flows, percentage		50.00%
Senior Notes \| 5.5% Senior Notes due 2024
Debt Instrument [Line Items]
Debt instrument, face amount		$ 425,000,000
Interest rate		5.50%
Subsequent Event \| Line of Credit \| Revolving Credit Facility \| Prior Four Quarter Period After Delivery of Financial Statements
Debt Instrument [Line Items]
Net leverage ratio	2
Subsequent Event \| Term Loan Facility \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	2.00%
Subsequent Event \| Term Loan Facility \| LIBOR \| Leverage Ratio is Met [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.75%
Subsequent Event \| Term Loan Facility \| Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate	1.00%
Subsequent Event \| Term Loan Facility \| Base Rate \| Leverage Ratio is Met [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	0.75%