Agreement extension term, period.

No definition available.

Name:	smhi_AgreementExtensionTermPeriod
Namespace Prefix:	smhi_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under a debt agreement on the amount that could be borrowed with a combination of, but not limited to, a line of credit and term loan.

+ References

No definition available.

+ Details

Name:	smhi_DebtAgreementMaximumBorrowingCapacity
Namespace Prefix:	smhi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

Description of the payment terms of the debt instrument (for example, whether periodic payments include principal and frequency of payments) and discussion about any contingencies associated with the payment.

+ 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(a)(3))
-URI http://asc.fasb.org/extlink&oid=120391452&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_DebtInstrumentPaymentTerms
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

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

- Definition

The cash outflow for debt initially having maturity due after one year or beyond the normal operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3291-108585

+ Details

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

- Details

Name:	us-gaap_GuaranteeObligationsByNatureAxis=smhi_LongtermDebtObligationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=smhi_FGUSAOmnibusAmendmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=smhi_TermAndRevolvingLoanFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=smhi_SeventhFGUSACreditFacilityAmendmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Long Term Debt - Additional Information (Details) - USD ($) $ in Thousands		3 Months Ended
	Feb. 24, 2021	Mar. 31, 2021	Mar. 31, 2020	Feb. 08, 2018
Debt Instrument [Line Items]
Repayments of long-term debt, total		$ 8,302	$ 5,069
Long-term Debt Obligation
Debt Instrument [Line Items]
Letters of credit outstanding, amount		$ 500
FGUSA Omnibus Amendment \| Term and Revolving Loan Facility
Debt Instrument [Line Items]
Debt agreement maximum borrowing capacity				$ 131,100
Seventh FGUSA Credit Facility Amendment
Debt Instrument [Line Items]
Repayment terms	the extension from March 2021 to June 2021 of the commencement of monthly repayment of the term loan, with payments being the lesser of (a) $0.8 million and (b) the amount outstanding under the term loan
Repayments of long-term debt, total	$ 800
Extension term of Guaranty	3 months