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Partners' Capital of the Operating Partnership - Additional Information (Details)	9 Months Ended
	Sep. 30, 2018 shares	Jun. 30, 2018 shares	Dec. 31, 2017 shares	Sep. 30, 2017 shares	Jun. 30, 2017 shares	Dec. 31, 2016 shares
Common Stock
Stockholders' Equity
Units outstanding (units)	85,594,000	85,725,000	92,803,000	97,446,000	98,367,000	100,562,000
SL Green Operating Partnership
Stockholders' Equity
Noncontrolling interest in the operating partnership (as a percent)	5.10%		4.58%
Number of units of operating partnership owned by the noncontrolling interest unit holders (units)	4,600,521		4,452,979
SL Green Operating Partnership | Series I Preferred Units
Stockholders' Equity
Units outstanding (units)	9,200,000
Period of restriction to redeem OP Units	1 year
Conversion of stock, shares issued	1
SL Green Operating Partnership | Common Stock
Stockholders' Equity
Units outstanding (units)	85,593,863

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4313-108586

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4332-108586

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-Name Accounting Standards Codification
-Topic 230
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