Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share [Abstract]
Income from continuing operations	$ 13,824	$ 12,410	$ 41,125	$ 38,008
Income from discontinued operations, net of income attributable to the noncontrolling interest	0	6,446	0	19,222
Net income attributable to Alexander's	$ 13,824	$ 18,856	$ 41,125	$ 57,230
Weighted average shares outstanding - basic and diluted (in shares)	5,109,717	5,108,016	5,108,832	5,107,474
Income from continuing operations (in dollars per share)	$ 2.71	$ 2.43	$ 8.05	$ 7.44
Income from discontinued operations, net (in dollars per share)	$ 0	$ 1.26	$ 0	$ 3.76
Net income per common share (in dollars per share)	$ 2.71	$ 3.69	$ 8.05	$ 11.20