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Transactions with Related Parties, Property Lease Agreement - 20F (Details) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2016 | Sep. 30, 2015 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Property Lease Agreement [Abstract] |  |  |  |  |
Expense from transactions with related party | $ 0 | $ 70 | $ 70 | $ 309 | $ 412
Waterfront S.A. [Member] | Rent Expense for Property Lease Agreement for Executive Offices [Member] |  |  |  |  |
Property Lease Agreement [Abstract] |  |  |  |  |
Number of months of rent guarantee |  |  |  | 3 months |
Rent guarantee |  |  |  | $ 55 |
Expense from transactions with related party | $ 0 | $ 70 | $ 70 | $ 309 | $ 412

X
- Definition Represents the number of months of rent guarantee. + References No definition available. + Details
Name: | ship_NumberOfMonthsOfRentGuarantee
Namespace Prefix: | ship_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition The rent guarantee for the rented offices. + References No definition available. + Details
Name: | ship_RentGuarantee
Namespace Prefix: | ship_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_RelatedPartyTransactionDueFromToRelatedPartyAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of selling, general and administrative expenses resulting from transactions, excluding transactions that are eliminated in consolidated or combined financial statements, with related party. + References No definition available. + Details
Name: | us-gaap_RelatedPartyTransactionSellingGeneralAndAdministrativeExpensesFromTransactionsWithRelatedParty
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=ship_WaterfrontSaMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RelatedPartyTransactionAxis=ship_RentExpenseForPropertyLeaseAgreementForExecutiveOfficesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |