Face amount of the debt instrument that is available for borrowing at time the agreement is executed and/or amended.

No definition available.

Name:	ship_DebtInstrumentBorrowingCapacity
Namespace Prefix:	ship_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The number of consecutive, periodic payment installments in which the debt is to be repaid.

+ References

No definition available.

+ Details

Name:	ship_DebtInstrumentNumberOfConsecutivePaymentInstallments
Namespace Prefix:	ship_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Period of time the debt instrument can be extended from the initial termination date, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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Namespace Prefix:	ship_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The pre-agreed sales price of the vessel M/V Lordship to the lender under the loan agreement on the fifth anniversary of the loan utilization (Year-5 Put Option).

+ References

No definition available.

+ Details

Name:	ship_PreAgreedSalesPriceOfVesselUnderYear5PutOption
Namespace Prefix:	ship_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The pre-agreed sales price of the vessel M/V Lordship to the lender under the loan agreement on the seventh anniversary of the loan utilization (Year-7 Put Option).

+ References

No definition available.

+ Details

Name:	ship_PreAgreedSalesPriceOfVesselUnderYear7PutOption
Namespace Prefix:	ship_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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

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

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_DebtInstrumentMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of payment greater than the preceding installment payments to be paid at final maturity date of debt.

+ References

No definition available.

+ Details

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

- Definition

Weighted average interest rate of long-term debt outstanding calculated over time.

+ References

No definition available.

+ Details

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

- Definition

The cash inflow from a debt initially having maturity due after one year or beyond the 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 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3255-108585

+ Details

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

- References

No definition available.

+ Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=ship_LoanAgreementDatedJune112018Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=ship_DebtInstrumentRedemptionDueJune2023Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentRedemptionPeriodAxis=ship_DebtInstrumentRedemptionDueJune2025Member
Namespace Prefix:
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Long-Term Debt and Financial Liabilities, Loan dated June 11, 2018 (Details) $ in Thousands		6 Months Ended
	Jun. 11, 2018 USD ($) Installment	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)
Secured Credit Facilities [Abstract]
Proceeds from draw down		$ 43,050	$ 18,000
Loan Agreement dated June 11, 2018 [Member]
Secured Credit Facilities [Abstract]
Borrowing capacity	$ 24,500
Pre-agreed sales price of vessel under Year-5 Put Option	20,800
Term of additional extension		2 years
Pre-agreed sales price of vessel under Year-7 Put Option	$ 15,000
Proceeds from draw down		$ 24,500
Loan Agreement dated June 11, 2018 [Member] \| Due June 2023 [Member]
Secured Credit Facilities [Abstract]
Maturity date		Jun. 30, 2023
Weighted average interest rate	11.40%
Number of consecutive payment installments \| Installment	20
Frequency of periodic payment		Quarterly
Balloon payment	$ 15,300
Loan Agreement dated June 11, 2018 [Member] \| Due June 2025 [Member]
Secured Credit Facilities [Abstract]
Maturity date		Jun. 30, 2025
Weighted average interest rate	11.20%
Number of consecutive payment installments \| Installment	28
Frequency of periodic payment		Quarterly
Balloon payment	$ 9,500