Stockholders' Equity of the Company - Additional Information (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2020 USD ($) increase $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Jun. 30, 2020 shares	Sep. 30, 2019 shares	Jun. 30, 2019 shares	Jun. 30, 2018 USD ($)	Aug. 31, 2016 USD ($)
Equity, Class of Treasury Stock [Line Items]
Authorized capital stock (shares)	260,000,000
Authorized shares, par value (in dollars per share) | $ / shares	$ 0.01
Common stock, shares authorized (in shares)	160,000,000	160,000,000
Common stock, par value (in dollars per share) | $ / shares	$ 0.01	$ 0.01
Excess stock, shares authorized (shares)	75,000,000
Excess stock, par value (in dollars per share) | $ / shares	$ 0.01
Preferred stock, shares authorized (shares)	25,000,000
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.01
Excess shares issued (shares)	0
Share repurchased (in shares)	6,243,165	4,596,171	9,744,911	8,342,411
Average price paid per share (in dollars per share) | $ / shares	$ 62.77	$ 83.62	$ 96.22	$ 101.64
2016 Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, authorized amount | $		$ 500,000,000.0	$ 500,000,000.0	$ 500,000,000.0				$ 500,000,000.0	$ 1,000,000,000.0
Number of increases to share repurchase program | increase	4
Share repurchased (in shares)	28,926,658	22,683,493	18,087,322	8,342,411
Stock repurchase program, authorized amount | $	$ 3,000,000,000.0
Settled In July 2020
Equity, Class of Treasury Stock [Line Items]
Share repurchased (in shares)	216,101
Common Stock
Equity, Class of Treasury Stock [Line Items]
Shares, outstanding (in shares)	73,040,000	79,202,000	83,684,000		73,675,000	81,515,000	82,410,000
SL Green Operating Partnership | Common Stock
Equity, Class of Treasury Stock [Line Items]
Shares, outstanding (in shares)	73,040,291