Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Earnings Per Share [Abstract]
Net income attributable to Alexander's	$ 20,634	$ 20,425	$ 20,157	$ 18,207	$ 17,891	$ 17,875	$ 15,549	$ 15,114	$ 79,423	$ 66,429	$ 132,190
Weighted average shares - basic									5,106,568	5,105,936	5,103,790
Dilutive effect of stock options									0	0	1,580
Weighted average shares - diluted									5,106,568	5,105,936	5,105,370
Net income per common share - basic	$ 4.04	$ 4.00	$ 3.95	$ 3.57	$ 3.50	$ 3.50	$ 3.05	$ 2.96	$ 15.55	$ 13.01	$ 25.90
Net income per common share - diluted	$ 4.04	$ 4.00	$ 3.95	$ 3.57	$ 3.50	$ 3.50	$ 3.05	$ 2.96	$ 15.55	$ 13.01	$ 25.89