Capital (Details 2) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Second Amended and Restated 2005 Stock Option and Incentive Plan
Second amended and restated 2005 stock option and incentive plan
Fungible units available for issuance (in shares)	0
SL Green
Summary of restricted stock
Compensation expense related to long-term compensation plans, restricted stock and stock options capitalized to assets	$ 1,000,000	$ 1,000,000
SL Green | Full-value awards
Second amended and restated 2005 stock option and incentive plan
Fungible units per share	1.65
SL Green | Stock options, stock appreciation rights and other awards
Second amended and restated 2005 stock option and incentive plan
Maximum fungible units that may be granted (in shares)	10,730,000
Fungible units per share	0.79
Award expiration period	5 years
SL Green | All other awards
Second amended and restated 2005 stock option and incentive plan
Fungible units per share	1.0
SL Green | Stock options
Second amended and restated 2005 stock option and incentive plan
Award expiration period	10 years
Weighted average assumptions used for estimation of fair value of each stock option
Dividend yield (as a percent)	2.00%		2.00%
Expected life of option	4 years 8 months 12 days		3 years 8 months 12 days
Risk-free interest rate (as a percent)	0.68%		0.46%
Expected stock price volatility (as a percent)	35.50%		37.40%
Options Outstanding
Balance at beginning of year (in shares)	1,201,000	1,277,200	1,277,200
Granted (in shares)	201,000		361,331
Exercised (in shares)	(79,036)		(382,612)
Lapsed or cancelled (in shares)	(4,000)		(54,919)
Balance at end of year (in shares)	1,318,964		1,201,000
Options exercisable at end of year (in shares)	585,786		479,913
Weighted average fair value of awards granted during the year	3,867,342		6,602,967
Weighted Average Exercise Price
Balance at beginning of year (in dollars per share)	$ 75.05	$ 63.37	$ 63.37
Granted (in dollars per share)	$ 76.67		$ 75.36
Exercised (in dollars per share)	$ 50.22		$ 36.65
Lapsed or cancelled (in dollars per share)	$ 67.64		$ 72.99
Balance at end of year (in dollars per share)	$ 76.81		$ 75.05
Options exercisable at end of year (in dollars per share)	$ 81.38		$ 86.85
Exercise price of options granted, low end of the range (in dollars per share)	$ 20.67
Exercise price of options granted, high end of the range (in dollars per share)	$ 137.18
Remaining weighted average contractual life of the options outstanding	4 years 5 months 12 days
Remaining weighted average contractual life of the options exercisable	3 years 9 months
Share-based compensation, additional disclosures
Compensation expense	1,300,000	1,700,000
Total unrecognized compensation cost related to unvested stock awards	11,600,000
Weighted average period for recognition of compensation cost related to unvested stock awards	3 years
SL Green | Stock options | Minimum
Second amended and restated 2005 stock option and incentive plan
Options vesting period, maximum	1 year
SL Green | Stock options | Maximum
Second amended and restated 2005 stock option and incentive plan
Options vesting period, maximum	5 years
SL Green | Restricted Stock Awards
Options Outstanding
Weighted average fair value of awards granted during the year			7,023,942
Share-based compensation, additional disclosures
Compensation expense	1,464,223		6,930,381
Total unrecognized compensation cost related to unvested stock awards	10,500,000
Weighted average period for recognition of compensation cost related to unvested stock awards	2 years 2 months 12 days
Annual award vesting rate, low end of the range (as a percent)	15.00%
Annual award vesting rate, high end of the range (as a percent)	35.00%
Summary of restricted stock
Balance at beginning of year (in shares)	2,804,901	2,912,456	2,912,456
Granted (in shares)			92,729
Cancelled (in shares)	(300)		(200,284)
Balance at end of year (in shares)	2,804,601		2,804,901
Vested during the year (in shares)	500		408,800
Fair value of restricted stock vested during the year	$ 20,000		$ 22,400,000
SL Green | Second Amended and Restated 2005 Stock Option and Incentive Plan
Second amended and restated 2005 stock option and incentive plan
Shares that may be issued if equal to fungible units	10,730,000