Consolidated Statements of Cash Flows (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating Activities
Net income	$ 545,863,000	$ 151,283,000	$ 209,700,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	400,001,000	357,599,000	351,539,000
Equity in net income from unconsolidated joint ventures	(26,537,000)	(9,921,000)	(76,418,000)
Distributions of cumulative earnings from unconsolidated joint ventures	28,859,000	34,997,000	91,145,000
Equity in net gain on sale of interest in unconsolidated joint venture interest/real estate	(123,253,000)	(3,601,000)	(37,053,000)
Purchase price fair value adjustment	(71,446,000)	2,305,000	0
Depreciable real estate reserves	0	2,150,000	0
Gain on sale of discontinued operations	(163,059,000)	(14,900,000)	(6,627,000)
Loan loss and other investment reserves, net of recoveries	0	0	564,000
Gain on sale of investment in marketable securities	(3,895,000)	0	(4,940,000)
Loss on early extinguishment of debt	32,365,000	10,963,000	6,978,000
Deferred rents receivable	(56,362,000)	(56,739,000)	(66,079,000)
Other non-cash adjustments	(28,559,000)	(37,843,000)	(18,868,000)
Changes in operating assets and liabilities:
Restricted cash—operations	861,000	2,037,000	(13,812,000)
Tenant and other receivables	1,978,000	(7,570,000)	(10,998,000)
Related party receivables	(3,673,000)	(917,000)	(3,529,000)
Deferred lease costs	(53,333,000)	(52,228,000)	(48,368,000)
Other assets	9,340,000	2,904,000	(35,932,000)
Accounts payable, accrued expenses and other liabilities and security deposits	(7,796,000)	(1,473,000)	9,389,000
Deferred revenue and land leases payable	9,027,000	7,157,000	62,000
Net cash provided by operating activities	490,381,000	386,203,000	346,753,000
Investing Activities
Acquisitions of real estate property	(1,039,530,000)	(594,595,000)	(544,568,000)
Additions to land, buildings and improvements	(369,887,000)	(196,571,000)	(148,148,000)
Escrowed cash—capital improvements/acquisition deposits	(8,461,000)	(7,672,000)	(70,080,000)
Investments in unconsolidated joint ventures	(382,010,000)	(150,274,000)	(215,174,000)
Distributions in excess of cumulative earnings from unconsolidated joint ventures	236,181,000	42,720,000	127,876,000
Proceeds from disposition of real estate/joint venture interest	820,599,000	227,615,000	178,485,000
Proceeds from sale of marketable securities	8,248,000	2,370,000	11,070,000
Purchases of marketable securities	(14,364,000)	(11,493,000)	(6,660,000)
Other investments	(7,448,000)	(43,163,000)	(36,750,000)
Origination of debt and preferred equity investments	(617,090,000)	(555,137,000)	(645,229,000)
Repayments or redemption of debt and preferred equity investments	576,927,000	657,765,000	185,775,000
Net cash used in investing activities	(796,835,000)	(628,435,000)	(1,163,403,000)
Financing Activities
Proceeds from mortgages and other loans payable	2,151,603,000	1,257,172,000	1,230,167,000
Repayments of mortgages and other loans payable	(2,201,999,000)	(1,085,220,000)	(684,954,000)
Proceeds from revolving credit facility and senior unsecured notes	1,908,000,000	1,164,000,000	1,751,480,000
Repayments of revolving credit facility and senior unsecured notes	(1,386,588,000)	(1,020,215,000)	(1,577,756,000)
Payments of debt extinguishment costs	(50,150,000)	0	(6,909,000)
Proceeds from stock options exercised and DRIP issuance	25,275,000	12,971,000	112,743,000
Net proceeds from sale of common stock	185,321,000	290,699,000	201,279,000
Net proceeds from sale of preferred stock	0	0	221,965,000
Redemption of preferred stock	(2,000,000)	(192,533,000)	(200,013,000)
Sale or purchase of treasury stock	0	7,120,000	(14,150,000)
Distributions to noncontrolling interests in other partnerships	(7,019,000)	(14,623,000)	(21,946,000)
Contributions from noncontrolling interests in other partnerships	30,675,000	8,164,000	25,894,000
Distributions to noncontrolling interests in the Operating Partnership	(7,849,000)	(4,146,000)	(3,296,000)
Dividends paid on common and preferred stock	(206,974,000)	(148,407,000)	(121,238,000)
Other obligations related to mortgage loan participations	0	0	5,000,000
Deferred loan costs and capitalized lease obligation	(57,124,000)	(16,042,000)	(49,824,000)
Net cash provided by financing activities	381,171,000	258,940,000	868,442,000
Net increase in cash and cash equivalents	74,717,000	16,708,000	51,792,000
Cash and cash equivalents at beginning of year	206,692,000	189,984,000	138,192,000
Cash and cash equivalents at end of year	281,409,000	206,692,000	189,984,000
Supplemental cash flow disclosures:
Interest paid	348,230,000	325,903,000	322,469,000
Income taxes paid	4,056,000	2,666,000	17,000
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Issuance of common stock as deferred compensation	1,601,000	164,000	722,000
Issuance of units in the Operating Partnership	56,469,000	24,750,000	42,239,000
Redemption of units in the Operating Partnership	31,653,000	17,287,000	87,513,000
Derivative instruments at fair value	11,230,000	636,000	92,000
Assignment of debt investment to joint venture	0	0	25,362,000
Mortgage assigned to joint venture	150,000,000	0	0
Tenant improvements and capital expenditures payable	9,408,000	502,000	1,738,000
Fair value adjustment to noncontrolling interest in the Operating Partnership	168,439,000	45,618,000	61,238,000
Assumption of mortgage loans	16,000,000	84,642,000	0
Consolidation of real estate investments and other adjustments	1,316,591,000	90,934,000	0
Repayment of mezzanine loans	0	0	13,750,000
Redemption of Series E units	0	0	31,698,000
Repayment of financing receivable	0	0	28,195,000
Investment in joint venture	88,957,000	0	5,135,000
Capital leased asset	0	9,992,000	28,132,000
Deconsolidation of a subsidiary	112,095,000	0	104,107,000
Transfer to net assets held for sale	462,430,000	0	4,901,000
Transfer to liabilities related to net assets held for sale	266,873,000	0	136,000
Transfer of financing receivable to debt investment	19,675,000	0	0
Deferred leasing payable	8,667,000	5,024,000	91,000
Issuance of preferred units	27,565,000	0	47,550,000
SL Green Operating Partnership
Operating Activities
Net income	545,863,000	151,283,000	209,700,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	400,001,000	357,599,000	351,539,000
Equity in net income from unconsolidated joint ventures	(26,537,000)	(9,921,000)	(76,418,000)
Distributions of cumulative earnings from unconsolidated joint ventures	28,859,000	34,997,000	91,145,000
Equity in net gain on sale of interest in unconsolidated joint venture interest/real estate	(123,253,000)	(3,601,000)	(37,053,000)
Purchase price fair value adjustment	(71,446,000)	2,305,000	0
Depreciable real estate reserves	0	2,150,000	0
Gain on sale of discontinued operations	(163,059,000)	(14,900,000)	(6,627,000)
Loan loss and other investment reserves, net of recoveries	0	0	564,000
Gain on sale of investment in marketable securities	(3,895,000)	0	(4,940,000)
Loss on early extinguishment of debt	32,365,000	10,963,000	6,978,000
Deferred rents receivable	(56,362,000)	(56,739,000)	(66,079,000)
Other non-cash adjustments	(28,559,000)	(37,843,000)	(18,868,000)
Changes in operating assets and liabilities:
Restricted cash—operations	861,000	2,037,000	(13,812,000)
Tenant and other receivables	1,978,000	(7,570,000)	(10,998,000)
Related party receivables	(3,673,000)	(917,000)	(3,529,000)
Deferred lease costs	(53,333,000)	(52,228,000)	(48,368,000)
Other assets	9,340,000	2,904,000	(35,932,000)
Accounts payable, accrued expenses and other liabilities and security deposits	(7,796,000)	(1,473,000)	9,389,000
Deferred revenue and land leases payable	9,027,000	7,157,000	62,000
Net cash provided by operating activities	490,381,000	386,203,000	346,753,000
Investing Activities
Acquisitions of real estate property	(1,039,530,000)	(594,595,000)	(544,568,000)
Additions to land, buildings and improvements	(369,887,000)	(196,571,000)	(148,148,000)
Escrowed cash—capital improvements/acquisition deposits	(8,461,000)	(7,672,000)	(70,080,000)
Investments in unconsolidated joint ventures	(382,010,000)	(150,274,000)	(215,174,000)
Distributions in excess of cumulative earnings from unconsolidated joint ventures	236,181,000	42,720,000	127,876,000
Proceeds from disposition of real estate/joint venture interest	820,599,000	227,615,000	178,485,000
Proceeds from sale of marketable securities	8,248,000	2,370,000	11,070,000
Purchases of marketable securities	(14,364,000)	(11,493,000)	(6,660,000)
Other investments	(7,448,000)	(43,163,000)	(36,750,000)
Origination of debt and preferred equity investments	(617,090,000)	(555,137,000)	(645,229,000)
Repayments or redemption of debt and preferred equity investments	576,927,000	657,765,000	185,775,000
Net cash used in investing activities	(796,835,000)	(628,435,000)	(1,163,403,000)
Financing Activities
Proceeds from mortgages and other loans payable	2,151,603,000	1,257,172,000	1,230,167,000
Repayments of mortgages and other loans payable	(2,201,999,000)	(1,085,220,000)	(684,954,000)
Proceeds from revolving credit facility and senior unsecured notes	1,908,000,000	1,164,000,000	1,751,480,000
Repayments of revolving credit facility and senior unsecured notes	(1,386,588,000)	(1,020,215,000)	(1,577,756,000)
Payments of debt extinguishment costs	(50,150,000)	0	(6,909,000)
Proceeds from stock options exercised and DRIP issuance	25,275,000	12,971,000	112,743,000
Net proceeds from sale of common stock	185,321,000	290,699,000	201,279,000
Net proceeds from sale of preferred stock	0	0	221,965,000
Redemption of preferred stock	(2,000,000)	(192,533,000)	(200,013,000)
Sale or purchase of treasury stock	0	7,120,000	(14,150,000)
Distributions to noncontrolling interests in other partnerships	(7,019,000)	(14,623,000)	(21,946,000)
Contributions from noncontrolling interests in other partnerships	30,675,000	8,164,000	25,894,000
Dividends paid on common and preferred stock	(214,823,000)	(152,553,000)	(124,534,000)
Other obligations related to mortgage loan participations	0	0	5,000,000
Deferred loan costs and capitalized lease obligation	(57,124,000)	(16,042,000)	(49,824,000)
Net cash provided by financing activities	381,171,000	258,940,000	868,442,000
Net increase in cash and cash equivalents	74,717,000	16,708,000	51,792,000
Cash and cash equivalents at beginning of year	206,692,000	189,984,000	138,192,000
Cash and cash equivalents at end of year	281,409,000	206,692,000	189,984,000
Supplemental cash flow disclosures:
Interest paid	348,230,000	325,903,000	322,469,000
Income taxes paid	4,056,000	2,666,000	17,000
Supplemental Disclosure of Non-Cash Investing and Financing Activities:
Issuance of common stock as deferred compensation	1,601,000	164,000	722,000
Issuance of units in the Operating Partnership	56,469,000	24,750,000	42,239,000
Redemption of units in the Operating Partnership	31,653,000	17,287,000	87,513,000
Derivative instruments at fair value	11,230,000	636,000	92,000
Assignment of debt investment to joint venture	0	0	25,362,000
Mortgage assigned to joint venture	150,000,000	0	0
Tenant improvements and capital expenditures payable	9,408,000	502,000	1,738,000
Assumption of mortgage loans	16,000,000	84,642,000	0
Consolidation of real estate investments and other adjustments	1,316,591,000	90,934,000	0
Repayment of mezzanine loans	0	0	13,750,000
Redemption of Series E units	0	0	31,698,000
Repayment of financing receivable	0	0	28,195,000
Investment in joint venture	88,957,000	0	5,135,000
Capital leased asset	0	9,992,000	28,132,000
Deconsolidation of a subsidiary	112,095,000	0	104,107,000
Transfer to net assets held for sale	462,430,000	0	4,901,000
Transfer to liabilities related to net assets held for sale	266,873,000	0	136,000
Transfer of financing receivable to debt investment	19,675,000	0	0
Deferred leasing payable	8,667,000	5,024,000	91,000
Issuance of preferred units	$ 27,565,000	$ 0	$ 47,550,000