Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Earnings Per Share [Abstract]
Income from continuing operations	$ 13,139	$ 12,566	$ 27,301	$ 25,598
Income from discontinued operations, net of income attributable to the noncontrolling interest	0	6,326	0	12,776
Net income attributable to Alexander's	$ 13,139	$ 18,892	$ 27,301	$ 38,374
Weighted average shares outstanding - basic and diluted (in shares)	5,108,745	5,107,415	5,108,383	5,107,199
Income from continuing operations (in dollars per share)	$ 2.57	$ 2.46	$ 5.34	$ 5.01
Income from discontinued operations, net (in dollars per share)	$ 0	$ 1.24	$ 0	$ 2.50
Net income per common share (in dollars per share)	$ 2.57	$ 3.70	$ 5.34	$ 7.51