Organization and Basis of Presentation (Details 2) (USD $) In Thousands, except Share data, unless otherwise specified	9 Months Ended
	Sep. 30, 2014 sqft building		Dec. 31, 2013
Real estate properties
Number of properties	99
Square Feet (sqft)	33,892,903
Weighted Average Occupancy (percent)	87.00%	[1]
Debt and preferred equity investments	$ 1,432,951		$ 1,304,839
Number of shares to be received on redemption of one unit of limited partnership interests (shares)	1
Commercial Property
Real estate properties
Number of properties	95
Square Feet (sqft)	33,090,705
Weighted Average Occupancy (percent)	86.80%	[1]
Residential
Real estate properties
Number of properties	4
Square Feet (sqft)	802,198
Weighted Average Occupancy (percent)	93.20%	[1]
Managed office properties
Real estate properties
Square Feet (sqft)	336,200
Consolidated properties
Real estate properties
Number of properties	76
Square Feet (sqft)	27,239,970
Consolidated properties | Commercial Property
Real estate properties
Number of properties	72
Square Feet (sqft)	26,437,772
Consolidated properties | Residential
Real estate properties
Number of properties	4
Square Feet (sqft)	802,198
Unconsolidated properties
Real estate properties
Number of properties	23
Square Feet (sqft)	6,652,933
Unconsolidated properties | Commercial Property
Real estate properties
Number of properties	23
Square Feet (sqft)	6,652,933
Unconsolidated properties | Residential
Real estate properties
Number of properties	0
Square Feet (sqft)	0
Manhattan
Real estate properties
Number of properties	61
Square Feet (sqft)	27,300,564
Weighted Average Occupancy (percent)	88.20%	[1]
Manhattan | Office
Real estate properties
Number of properties	30
Square Feet (sqft)	21,905,160
Weighted Average Occupancy (percent)	95.40%	[1]
Manhattan | Retail
Real estate properties
Number of properties	15
Square Feet (sqft)	677,830
Weighted Average Occupancy (percent)	90.30%	[1]
Manhattan | Development
Real estate properties
Number of properties	13
Square Feet (sqft)	3,579,644
Weighted Average Occupancy (percent)	39.70%	[1]
Manhattan | Fee Interest
Real estate properties
Number of properties	3
Square Feet (sqft)	1,137,930
Weighted Average Occupancy (percent)	100.00%	[1]
Manhattan | Residential
Real estate properties
Number of properties	3
Square Feet (sqft)	735,587
Weighted Average Occupancy (percent)	94.10%	[1]
Manhattan | Consolidated properties
Real estate properties
Number of properties	43
Square Feet (sqft)	21,935,372
Manhattan | Consolidated properties | Office
Real estate properties
Number of properties	23
Square Feet (sqft)	18,429,045
Manhattan | Consolidated properties | Retail
Real estate properties
Number of properties	8	[2]
Square Feet (sqft)	394,535
Manhattan | Consolidated properties | Development
Real estate properties
Number of properties	9	[3]
Square Feet (sqft)	1,973,862
Manhattan | Consolidated properties | Fee Interest
Real estate properties
Number of properties	3	[3]
Square Feet (sqft)	1,137,930
Manhattan | Consolidated properties | Residential
Real estate properties
Number of properties	3	[2]
Square Feet (sqft)	735,587
Manhattan | Consolidated properties | Dual Property Type, Residential Portion
Real estate properties
Square Feet (sqft)	222,855
Manhattan | Consolidated properties | Dual Property Type, Retail Portion
Real estate properties
Square Feet (sqft)	270,132
Manhattan | Unconsolidated properties
Real estate properties
Number of properties	18
Square Feet (sqft)	5,365,192
Manhattan | Unconsolidated properties | Office
Real estate properties
Number of properties	7
Square Feet (sqft)	3,476,115
Manhattan | Unconsolidated properties | Retail
Real estate properties
Number of properties	7
Square Feet (sqft)	283,295
Manhattan | Unconsolidated properties | Development
Real estate properties
Number of properties	4
Square Feet (sqft)	1,605,782
Manhattan | Unconsolidated properties | Fee Interest
Real estate properties
Number of properties	0
Square Feet (sqft)	0
Manhattan | Unconsolidated properties | Residential
Real estate properties
Number of properties	0
Square Feet (sqft)	0
Suburban
Real estate properties
Number of properties	34
Square Feet (sqft)	5,790,141
Weighted Average Occupancy (percent)	80.70%	[1]
Suburban | Office
Real estate properties
Number of properties	31
Square Feet (sqft)	5,587,500
Weighted Average Occupancy (percent)	81.30%	[1]
Suburban | Retail
Real estate properties
Number of properties	1
Square Feet (sqft)	52,000
Weighted Average Occupancy (percent)	100.00%	[1]
Suburban | Development
Real estate properties
Number of properties	2
Square Feet (sqft)	150,641
Weighted Average Occupancy (percent)	50.50%	[1]
Suburban | Residential
Real estate properties
Number of properties	1
Square Feet (sqft)	66,611
Weighted Average Occupancy (percent)	79.20%	[1]
Suburban | Consolidated properties
Real estate properties
Number of properties	29
Square Feet (sqft)	4,502,400
Suburban | Consolidated properties | Office
Real estate properties
Number of properties	27
Square Feet (sqft)	4,365,400
Suburban | Consolidated properties | Retail
Real estate properties
Number of properties	1
Square Feet (sqft)	52,000
Suburban | Consolidated properties | Development
Real estate properties
Number of properties	1
Square Feet (sqft)	85,000
Suburban | Consolidated properties | Residential
Real estate properties
Number of properties	1
Square Feet (sqft)	66,611
Suburban | Unconsolidated properties
Real estate properties
Number of properties	5
Square Feet (sqft)	1,287,741
Suburban | Unconsolidated properties | Office
Real estate properties
Number of properties	4
Square Feet (sqft)	1,222,100
Suburban | Unconsolidated properties | Retail
Real estate properties
Number of properties	0
Square Feet (sqft)	0
Suburban | Unconsolidated properties | Development
Real estate properties
Number of properties	1
Square Feet (sqft)	65,641
Suburban | Unconsolidated properties | Residential
Real estate properties
Number of properties	0
Square Feet (sqft)	0

[1] The weighted average occupancy for commercial properties represents the total occupied square feet divided by total available rentable square feet. The weighted average occupancy for residential properties represents the total occupied units divided by total available units.

[2] As of September 30, 2014, we owned a building that was comprised of 270,132 square feet of retail space and 222,855 square feet of residential space. For the purpose of this report, we have included the building as part of retail properties and have shown the square footage under its respective classifications.

[3] Includes two properties, which are held for sale as of September 30, 2014.