Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Earnings Per Share [Abstract]
Income from continuing operations	$ 12,410	$ 14,941	$ 38,008	$ 41,513
Income from discontinued operations, net of income attributable to the noncontrolling interest	6,446	5,484	19,222	17,276
Net income attributable to Alexander's	$ 18,856	$ 20,425	$ 57,230	$ 58,789
Weighted average shares outstanding - basic	5,108,016	5,106,984	5,107,474	5,106,427
Weighted average shares outstanding - diluted (in shares)	5,108,016	5,106,984	5,107,474	5,106,427
Income from continuing operations - basic (in dollars per share)	$ 2.43	$ 2.93	$ 7.44	$ 8.13
Income from discontinued operations, net - basic (in dollars per share)	$ 1.26	$ 1.07	$ 3.76	$ 3.38
Net income per common share - basic (in dollars per share)	$ 3.69	$ 4.00	$ 11.20	$ 11.51
Income from continuing operations - diluted (in dollars per share)	$ 2.43	$ 2.93	$ 7.44	$ 8.13
Income from discontinued operations, net - diluted (in dollars per share)	$ 1.26	$ 1.07	$ 3.76	$ 3.38
Net income per common share - diluted (in dollars per share)	$ 3.69	$ 4.00	$ 11.20	$ 11.51