Organization and Business (Details)	12 Months Ended
	Dec. 31, 2012
Organization And Business Details [Abstract]
Common limited partnership interest in the Operating Partnership	94.00%
JC Penney [Member]
Real Estate Properties [Line Items]
Equity method ownership percentage	10.70%
Real Estate Investment Fund [Member]
Real Estate Properties [Line Items]
Equity method ownership percentage	25.00%
Toys R Us [Member]
Real Estate Properties [Line Items]
Equity method ownership percentage	32.60%
Washington DC Segment [Member] | Washington Dc Northern Virginia [Member]
Real Estate Properties [Line Items]
Number Of Real Estate Properties	73
Square Footage Of Real Estate Property	19,100,000
Alexanders Inc [Member]
Real Estate Properties [Line Items]
Equity method ownership percentage	32.40%
Office Properties [Member] | San Francisco [Member] | Other Properties 555 California Street [Member]
Real Estate Properties [Line Items]
Number Of Real Estate Properties	3
Square Footage Of Real Estate Property	1,800,000
Contolling Interest In Real Estate Properties Percent	70.00%
Office Properties [Member] | New York Segment [Member] | Midtown Manhattan [Member]
Real Estate Properties [Line Items]
Number Of Real Estate Properties	31
Office Properties [Member] | New York Segment [Member] | Manhattan [Member]
Real Estate Properties [Line Items]
Square Footage Of Real Estate Property	19,700,000
Office Properties [Member] | Washington DC Segment [Member] | Washington Dc Northern Virginia [Member]
Real Estate Properties [Line Items]
Number Of Real Estate Properties	59
Square Footage Of Real Estate Property	16,100,000
Retail Properties [Member] | New York Segment [Member] | Manhattan [Member]
Real Estate Properties [Line Items]
Number Of Real Estate Properties	49
Square Footage Of Real Estate Property	2,200,000
Retail Properties [Member] | Retail Properties Segment [Member] | Northeast States And California [Member]
Real Estate Properties [Line Items]
Number Of Real Estate Properties	114
Square Footage Of Real Estate Property	15,600,000
Retail Properties [Member] | Retail Properties Segment [Member] | Northeast Mid Atlantic States And Puerto Rico [Member]
Real Estate Properties [Line Items]
Number Of Real Estate Properties	6
Square Footage Of Real Estate Property	5,200,000
Merchandise Mart Properties [Member] | Chicago [Member] | Merchandise Mart IL Chicago [Member]
Real Estate Properties [Line Items]
Square Footage Of Real Estate Property	3,500,000
Other Real Estate And Related Investments [Member] | New York Segment [Member] | Manhattan [Member] | Hotel Pennsylvania [Member]
Real Estate Properties [Line Items]
Number of Units in Real Estate Property	1,700
Other Real Estate And Related Investments [Member] | Alexanders Inc [Member] | New York City Metropolitan Area [Member]
Real Estate Properties [Line Items]
Number Of Real Estate Properties	6
Other Real Estate And Related Investments [Member] | Alexanders Inc [Member] | New York City Metropolitan Area [Member] | Lexington Avenue 731 [Member]
Real Estate Properties [Line Items]
Square Footage Of Real Estate Property	1,300,000
Residential Properties [Member] | New York Segment [Member] | Manhattan [Member]
Real Estate Properties [Line Items]
Number Of Real Estate Properties	4
Number of Units in Real Estate Property	1,655
Residential Properties [Member] | Washington DC Segment [Member] | Washington Dc Northern Virginia [Member]
Real Estate Properties [Line Items]
Number Of Real Estate Properties	7
Number of Units in Real Estate Property	2,414