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

No definition available.

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Balance Type:	na
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- Definition

Amount of cash deposit to be maintained in a sale and leaseback transaction involving the sale of property to another party and the lease of the property back to the seller.

+ References

No definition available.

+ Details

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

- Definition

Amount of deposit paid in a transaction involving the sale of property to another party and the lease of the property back to the seller.

+ References

No definition available.

+ Details

Name:	ship_SaleLeasebackAgreementDeposit
Namespace Prefix:	ship_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Market value of the asset in a sale-leaseback agreement.

+ References

No definition available.

+ Details

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

- Definition

Term of charter on a bareboat basis under the sale leaseback transaction, 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

Amount of cash payment made to the owner under the sale leaseback agreement upon the delivery of the vessel.

+ References

No definition available.

+ Details

Name:	ship_SaleLeasebackAgreementUpfrontCharterhirePayment
Namespace Prefix:	ship_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The minimum threshold value maintenance ratio (expressed as a percentage) as defined in the additional clauses of the bareboat charter to be maintained by the Company.

+ References

No definition available.

+ Details

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Namespace Prefix:	ship_
Data Type:	xbrli:pureItemType
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Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

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

- Definition

Amount of the required periodic payments applied to principal.

+ References

+ Details

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

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

- References

No definition available.

+ Details

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

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

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
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Long-Term Debt and Financial Liabilities, Failed Sale and Leaseback Agreement (Financial Liability) (Details) $ in Thousands		6 Months Ended
	Jun. 28, 2018 USD ($) Installment	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Secured Credit Facilities [Abstract]
Deposit made under sale and leaseback agreement	$ 1,325	$ 1,325	$ 0
Principal amount outstanding		$ 200,508		$ 196,450
Sale and Leaseback Agreement [Member]
Secured Credit Facilities [Abstract]
Sale and leaseback	26,500
Term of charter contract		8 years
Upfront charterhire payment	6,625
Deposit made under sale and leaseback agreement	$ 1,325
Minimum value maintenance ratio to be maintained	1.2
Cash deposit to be maintained	$ 1,325
Number of consecutive payment installments \| Installment	32
Frequency of periodic payment		Quarterly
Installment payment	$ 456
Balloon payment	$ 5,299
Principal amount outstanding		$ 19,875
Sale and Leaseback Agreement [Member] \| LIBOR [Member]
Secured Credit Facilities [Abstract]
Margin on variable rate	4.00%