Quarterly Financial Data of the Operating Partnership (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Income Statements, Captions [Line Items]
Total revenues	$ 308,082	$ 313,634	$ 313,024	$ 304,255	$ 317,036	$ 307,545	$ 301,116	$ 301,695	$ 1,238,995	$ 1,227,392	$ 1,511,473
Total expenses	(252,712)	(263,341)	(258,383)	(260,328)	(267,678)	(265,553)	(258,303)	(258,282)	(1,034,764)	(1,049,816)	(1,346,445)
Equity in net (loss) income from unconsolidated joint ventures	(11,874)	(9,864)	(7,546)	(5,234)	(2,398)	971	4,702	4,036	(34,518)	7,311	21,892
Equity in net gain on sale of interest in unconsolidated joint venture/real estate	0	0	59,015	17,166	167,445	70,937	72,025	(6,440)	76,181	303,967	16,166
(Loss) gain on sale of real estate, net	(19,241)	3,541	0	(1,049)	(36,984)	(2,504)	(14,790)	23,521	(16,749)	(30,757)	73,241
Purchase price and other fair value adjustment	0	3,799	67,631	(2,041)	0	(3,057)	11,149	49,293	69,389	57,385	0
Depreciable real estate reserves and impairment	0	(7,047)	0	0	(220,852)	(6,691)	0	0	(7,047)	(227,543)	(178,520)
Gain on sale of investment in marketable securities									0	0	3,262
Loss on early extinguishment of debt					(14,889)	(2,194)	0	0	0	(17,083)	0
Noncontrolling interests and preferred unit distributions	(3,086)	(3,827)	(8,901)	(5,239)	838	(7,507)	(8,606)	(8,319)
Net income (loss) attributable to SL Green/SLGOP	21,169	36,895	164,840	47,530	(57,482)	91,947	107,293	105,504	270,434	247,262	101,374
SL Green Operating Partnership
Condensed Income Statements, Captions [Line Items]
Total revenues	308,082	313,634	313,024	304,255	317,036	307,545	301,116	301,695	1,238,995	1,227,392	1,511,473
Total expenses	(252,713)	(263,341)	(258,383)	(260,328)	(267,678)	(265,553)	(258,303)	(258,282)	(1,034,764)	(1,049,816)	(1,346,445)
Equity in net (loss) income from unconsolidated joint ventures	(11,874)	(9,864)	(7,546)	(5,234)	(2,398)	971	4,702	4,036	(34,518)	7,311	21,892
Equity in net gain on sale of interest in unconsolidated joint venture/real estate	0	0	59,015	17,166	167,445	70,937	72,025	(6,440)	76,181	303,967	16,166
(Loss) gain on sale of real estate, net	(19,241)	3,541	0	(1,049)	(36,984)	(2,504)	(14,790)	23,521	(16,749)	(30,757)	73,241
Purchase price and other fair value adjustment	0	3,799	67,631	(2,041)	0	(3,057)	11,149	49,293	69,389	57,385	0
Depreciable real estate reserves and impairment	0	(7,047)	0	0					(7,047)	(227,543)	(178,520)
Gain on sale of investment in marketable securities					(220,852)	(6,691)	0	0	0	0	3,262
Loss on early extinguishment of debt					(14,889)	(2,194)	0	0	0	(17,083)	0
Noncontrolling interests and preferred unit distributions	(2,091)	(2,108)	(591)	(2,961)	(2,601)	(2,710)	(3,020)	(3,047)	(14,950)	(14,950)	(14,950)
Net income (loss) attributable to SL Green/SLGOP	22,163	38,614	173,150	49,808	(60,921)	96,744	112,879	110,776	283,735	259,478	105,369
Perpetual preferred stock dividends	(3,737)	(3,738)	(3,737)	(3,738)	(3,737)	(3,738)	(3,737)	(3,738)
Net income attributable to SLGOP common unitholders	$ 18,426	$ 34,876	$ 169,413	$ 46,070	$ (64,658)	$ 93,006	$ 109,142	$ 107,038	$ 268,785	$ 244,528	$ 90,419
Net income attributable to common unitholders per common share—basic (in dollars per share)	$ 0.21	$ 0.40	$ 1.94	$ 0.52	$ (0.73)	$ 1.03	$ 1.19	$ 1.12	$ 3.10	$ 2.67	$ 0.87
Net income attributable to common unitholders per common share—diluted (in dollars per share)	$ 0.21	$ 0.40	$ 1.94	$ 0.52	$ (0.73)	$ 1.03	$ 1.19	$ 1.12	$ 3.10	$ 2.67	$ 0.87