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Share-based Compensation - Additional Information (Details) fungibleUnit in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2021 USD ($) unit / shares unit fungibleUnit shares | Sep. 30, 2020 USD ($) | Sep. 30, 2021 USD ($) unit / shares unit fungibleUnit shares | Sep. 30, 2020 USD ($) | Dec. 31, 2020 USD ($) shares | Dec. 31, 2019 shares | Jan. 01, 2008 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Capitalized to assets associated with compensation expense related to our long-term compensation plans, restricted stock and stock options | $ 500,000 | $ 600,000 | $ 1,600,000 | $ 1,700,000 |  |  |
Stock options |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Period of commencement of option vesting, from date of grant (in years) |  |  | 1 year |  |  |  |
Options granted (in shares) | shares |  |  | 0 |  | 0 |  |
Remaining weighted average contractual life of the options outstanding (in years) |  |  | 1 year 7 months 6 days |  |  |  |
Remaining average contractual life of the options exercisable (in years) |  |  | 1 year 7 months 6 days |  |  |  |
Stock options | Minimum |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Award expiration period (in years) |  |  | 5 years |  |  |  |
Options vesting period (in years) |  |  | 1 year |  |  |  |
Stock options | Maximum |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Award expiration period (in years) |  |  | 10 years |  |  |  |
Options vesting period (in years) |  |  | 5 years |  |  |  |
Class O LTIP Units |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Preferred unit distributions as a percentage of common unit distributions | 10.00% |  | 10.00% |  |  |  |
Restricted Stock Awards |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Compensation expense |  |  | $ 6,339,292 |  | $ 10,895,459 |  |
Total unrecognized compensation cost related to unvested stock awards | $ 9,100,000 |  | $ 9,100,000 |  |  |  |
Annual award vesting rate, low end of range (as a percent) |  |  | 15.00% |  |  |  |
Annual award vesting rate, high end of range (as a percent) |  |  | 35.00% |  |  |  |
Fair value of restricted stock vested during the period |  |  | $ 11,300,000 |  | 12,500,000 |  |
Weighted average period for recognition of compensation cost related to unvested stock awards (in years) |  |  | 1 year 9 months 18 days |  |  |  |
Weighted average fair value of options granted during the period |  |  | $ 9,214,531 |  | $ 734,315 |  |
Awards granted (in shares) | shares |  |  | 145,845 |  | 8,959 |  |
Awards outstanding (in shares) | shares | 3,566,283 |  | 3,566,283 |  | 3,439,674 | 3,465,347 |
LTIP units |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Weighted average fair value of options granted during the period |  |  | $ 45,100,000 |  | $ 37,000,000 |  |
Performance Shares |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Compensation expense | $ 9,100,000 | 6,400,000 | 27,300,000 | 20,400,000 |  |  |
Total unrecognized compensation cost related to unvested stock awards | $ 51,200,000 |  | $ 51,200,000 |  |  |  |
Weighted average period for recognition of compensation cost related to unvested stock awards (in years) |  |  | 1 year 8 months 12 days |  |  |  |
All other awards |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Expected term |  |  | 10 years |  |  |  |
Third Amendment and Restated 2005 Stock Option and Incentive Plan |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Maximum fungible units that may be granted (in shares) | unit | 27,030,000 |  | 27,030,000 |  |  |  |
Fungible units per share (in fungible units per share) | unit / shares | 3.74 |  | 3.74 |  |  |  |
Shares that may be issued if equal to fungible units (shares) (less than) | shares | 27,030,000 |  | 27,030,000 |  |  |  |
Fungible units | fungibleUnit | 1.4 |  | 1.4 |  |  |  |
Third Amendment and Restated 2005 Stock Option and Incentive Plan | Stock options |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Compensation expense | $ 0 | 0 | $ 0 | 30,000.00 |  |  |
Total unrecognized compensation cost related to unvested stock awards | $ 0 |  | $ 0 |  |  |  |
Stock options, stock appreciation rights and other awards |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Fungible units per share (in fungible units per share) | unit / shares | 0.73 |  | 0.73 |  |  |  |
Award expiration period (in years) |  |  | 5 years |  |  |  |
All other awards |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Fungible units per share (in fungible units per share) | unit / shares | 1.0 |  | 1.0 |  |  |  |
Deferred Stock Compensation Plan for Directors |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Compensation expense | $ 100,000 | $ 200,000 | $ 2,200,000 | $ 2,100,000 |  |  |
Maximum percentage of the annual retainer fee, chairman fees and meeting fees that may be deferred by non-employee directors (percent) | 100.00% |  | 100.00% |  |  |  |
Awards granted (in shares) | shares |  |  | 22,413 |  |  |  |
Shares issued (in shares) | shares |  |  | 12,226 |  |  |  |
Awards outstanding (in shares) | shares | 163,189 |  | 163,189 |  |  |  |
Employee Stock Purchase Plan |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Shares of common stock available for issuance (shares) | shares |  |  |  |  |  |  | 500,000
Duration of each offering period starting the first day of each calendar quarter (in months) |  |  | 3 months |  |  |  |
Discount from market price |  |  | 85.00% |  |  |  |
Shares of common stock issued (shares) | shares | 169,802 |  | 169,802 |  |  |  |

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Name: | us-gaap_PlanNameAxis=slg_EmployeeStockPurchasePlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |