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Deferred Costs (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
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Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract] |  |
Deferred leasing costs | $ 438,964 | $ 443,341
Less: accumulated amortization | (236,464) | (217,140)
Deferred costs, net | $ 202,500 | $ 226,201

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- Definition The accumulated amortization, as of the reporting date, representing the periodic charge to earnings of deferred costs. + References No definition available. + Details
Name: | slg_AccumulatedAmortizationOfDeferredCosts
Namespace Prefix: | slg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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- Definition The carrying amount of deferred costs. + References No definition available. + Details
Name: | us-gaap_DeferredCostsCurrentAndNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition For an unclassified balance sheet, this element represents costs incurred by the lessor that are (a) costs to originate a lease incurred in transactions with independent third parties that (i) result directly from and are essential to acquire that lease and (ii) would not have been incurred had that leasing transaction not occurred and (b) certain costs directly related to specified activities performed by the lessor for that lease. Those activities are: evaluating the prospective lessee's financial condition; evaluating and recording guarantees, collateral, and other security arrangements; negotiating lease terms; preparing and processing lease documents; and closing the transaction. This amount is before considering accumulated amortization representing the periodic charge to earnings to recognize the deferred costs over the term of the related lease. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 25 -Paragraph 16 -URI http://asc.fasb.org/extlink&oid=77902619&loc=d3e40588-112709 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 25 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=77902619&loc=d3e40246-112709 + Details
Name: | us-gaap_DeferredCostsLeasingGross
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant