Organization and Basis of Presentation (Details 2) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 sqft building		Dec. 31, 2013
Real estate properties
Number of properties (buildings)	101
Approximate Square Feet (sqft)	33,891,036
Weighted Average Occupancy (as a percent)	85.80%	[1]
Carrying Value, Net of Discounts and Deferred Origination Fees	$ 1,408,804		$ 1,304,839
Number of shares to be received on redemption of one unit of limited partnership interests (shares)	1
Residential
Real estate properties
Number of properties (buildings)	4
Approximate Square Feet (sqft)	802,198
Weighted Average Occupancy (as a percent)	95.20%	[1]
Managed office properties
Real estate properties
Approximate Square Feet (sqft)	336,201
Commercial properties
Real estate properties
Number of properties (buildings)	97
Approximate Square Feet (sqft)	33,088,838
Weighted Average Occupancy (as a percent)	85.50%	[1]
Consolidated properties
Real estate properties
Number of properties (buildings)	76
Approximate Square Feet (sqft)	26,894,770
Consolidated properties | Residential
Real estate properties
Number of properties (buildings)	4
Approximate Square Feet (sqft)	802,198
Consolidated properties | Commercial properties
Real estate properties
Number of properties (buildings)	72
Approximate Square Feet (sqft)	26,092,572
Unconsolidated properties
Real estate properties
Number of properties (buildings)	25
Approximate Square Feet (sqft)	6,996,266
Unconsolidated properties | Residential
Real estate properties
Number of properties (buildings)	0
Approximate Square Feet (sqft)	0
Unconsolidated properties | Commercial properties
Real estate properties
Number of properties (buildings)	25
Approximate Square Feet (sqft)	6,996,266
Manhattan
Real estate properties
Number of properties (buildings)	62
Approximate Square Feet (sqft)	27,298,697
Weighted Average Occupancy (as a percent)	86.30%	[1]
Manhattan | Office
Real estate properties
Number of properties (buildings)	30
Approximate Square Feet (sqft)	21,905,160
Weighted Average Occupancy (as a percent)	95.30%	[1]
Manhattan | Retail
Real estate properties
Number of properties (buildings)	16
Approximate Square Feet (sqft)	683,363
Weighted Average Occupancy (as a percent)	91.00%	[1]
Manhattan | Development/Redevelopment
Real estate properties
Number of properties (buildings)	14
Approximate Square Feet (sqft)	3,926,644
Weighted Average Occupancy (as a percent)	32.60%	[1]
Manhattan | Fee interest
Real estate properties
Number of properties (buildings)	2
Approximate Square Feet (sqft)	783,530
Weighted Average Occupancy (as a percent)	100.00%	[1]
Manhattan | Residential
Real estate properties
Number of properties (buildings)	3
Approximate Square Feet (sqft)	735,587
Weighted Average Occupancy (as a percent)	95.60%	[1]
Manhattan | Consolidated properties
Real estate properties
Number of properties (buildings)	43
Approximate Square Feet (sqft)	21,590,172
Manhattan | Consolidated properties | Office
Real estate properties
Number of properties (buildings)	23
Approximate Square Feet (sqft)	18,429,045
Manhattan | Consolidated properties | Retail
Real estate properties
Number of properties (buildings)	9	[2]
Approximate Square Feet (sqft)	403,735
Manhattan | Consolidated properties | Development/Redevelopment
Real estate properties
Number of properties (buildings)	9	[3]
Approximate Square Feet (sqft)	1,973,862
Manhattan | Consolidated properties | Fee interest
Real estate properties
Number of properties (buildings)	2
Approximate Square Feet (sqft)	783,530
Manhattan | Consolidated properties | Residential
Real estate properties
Number of properties (buildings)	3	[2]
Approximate Square Feet (sqft)	735,587
Manhattan | Consolidated properties | Dual property type, retail portion
Real estate properties
Approximate Square Feet (sqft)	270,132
Manhattan | Consolidated properties | Dual property type, residential portion
Real estate properties
Approximate Square Feet (sqft)	222,855
Manhattan | Unconsolidated properties
Real estate properties
Number of properties (buildings)	19
Approximate Square Feet (sqft)	5,708,525
Manhattan | Unconsolidated properties | Office
Real estate properties
Number of properties (buildings)	7
Approximate Square Feet (sqft)	3,476,115
Manhattan | Unconsolidated properties | Retail
Real estate properties
Number of properties (buildings)	7
Approximate Square Feet (sqft)	279,628
Manhattan | Unconsolidated properties | Development/Redevelopment
Real estate properties
Number of properties (buildings)	5
Approximate Square Feet (sqft)	1,952,782
Manhattan | Unconsolidated properties | Fee interest
Real estate properties
Number of properties (buildings)	0
Approximate Square Feet (sqft)	0
Manhattan | Unconsolidated properties | Residential
Real estate properties
Number of properties (buildings)	0
Approximate Square Feet (sqft)	0
Suburban
Real estate properties
Number of properties (buildings)	35
Approximate Square Feet (sqft)	5,790,141
Weighted Average Occupancy (as a percent)	81.80%	[1]
Suburban | Office
Real estate properties
Number of properties (buildings)	31
Approximate Square Feet (sqft)	5,587,500
Weighted Average Occupancy (as a percent)	82.40%	[1]
Suburban | Retail
Real estate properties
Number of properties (buildings)	1
Approximate Square Feet (sqft)	52,000
Weighted Average Occupancy (as a percent)	100.00%	[1]
Suburban | Development/Redevelopment
Real estate properties
Number of properties (buildings)	3
Approximate Square Feet (sqft)	150,641
Weighted Average Occupancy (as a percent)	54.20%	[1]
Suburban | Residential
Real estate properties
Number of properties (buildings)	1
Approximate Square Feet (sqft)	66,611
Weighted Average Occupancy (as a percent)	89.60%	[1]
Suburban | Consolidated properties
Real estate properties
Number of properties (buildings)	29
Approximate Square Feet (sqft)	4,502,400
Suburban | Consolidated properties | Office
Real estate properties
Number of properties (buildings)	27
Approximate Square Feet (sqft)	4,365,400
Suburban | Consolidated properties | Retail
Real estate properties
Number of properties (buildings)	1
Approximate Square Feet (sqft)	52,000
Suburban | Consolidated properties | Development/Redevelopment
Real estate properties
Number of properties (buildings)	1
Approximate Square Feet (sqft)	85,000
Suburban | Consolidated properties | Residential
Real estate properties
Number of properties (buildings)	1
Approximate Square Feet (sqft)	66,611
Suburban | Unconsolidated properties
Real estate properties
Number of properties (buildings)	6
Approximate Square Feet (sqft)	1,287,741
Suburban | Unconsolidated properties | Office
Real estate properties
Number of properties (buildings)	4
Approximate Square Feet (sqft)	1,222,100
Suburban | Unconsolidated properties | Retail
Real estate properties
Number of properties (buildings)	0
Approximate Square Feet (sqft)	0
Suburban | Unconsolidated properties | Development/Redevelopment
Real estate properties
Number of properties (buildings)	2
Approximate Square Feet (sqft)	65,641
Suburban | Unconsolidated properties | Residential
Real estate properties
Number of properties (buildings)	0
Approximate 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 December 31, 2014, we owned a building that was comprised of approximately 270,132 square feet (unaudited) of retail space and approximately 222,855 square feet (unaudited) 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 one property which was held for sale as of December 31, 2014 and sold in January 2015.