Organization and Basis of Presentation (Details 2) $ in Thousands	9 Months Ended
	Sep. 30, 2015 USD ($) ft² building property shares	Dec. 31, 2014 USD ($)
Real estate properties
Number of Properties | building	118
Approximate Square Feet (sqft)	35,843,296
Weighted Average Occupancy (as a percent)	90.20%
Carrying Value, Net of Discounts and Deferred Origination Fees | $	$ 1,501,619	$ 1,408,804
Number of shares to be received on redemption of one unit of limited partnership interests (shares) | shares	1
Commercial properties
Real estate properties
Number of Properties | building	95
Approximate Square Feet (sqft)	32,803,865
Weighted Average Occupancy (as a percent)	89.80%
Residential
Real estate properties
Number of Properties | building	23
Approximate Square Feet (sqft)	3,039,431
Weighted Average Occupancy (as a percent)	94.60%
Managed office properties
Real estate properties
Approximate Square Feet (sqft)	336,201
Consolidated properties
Real estate properties
Number of Properties | building	77
Approximate Square Feet (sqft)	27,298,174
Consolidated properties | Commercial properties
Real estate properties
Number of Properties | building	72
Approximate Square Feet (sqft)	26,468,976
Consolidated properties | Residential
Real estate properties
Number of Properties | building	5
Approximate Square Feet (sqft)	829,198
Unconsolidated properties
Real estate properties
Number of Properties | building	41
Approximate Square Feet (sqft)	8,545,122
Unconsolidated properties | Commercial properties
Real estate properties
Number of Properties | building	23
Approximate Square Feet (sqft)	6,334,889
Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	18
Approximate Square Feet (sqft)	2,210,233
Manhattan
Real estate properties
Number of Properties | building	61
Approximate Square Feet (sqft)	27,594,824
Weighted Average Occupancy (as a percent)	91.50%
Manhattan | Office
Real estate properties
Number of Properties | building	32
Approximate Square Feet (sqft)	24,028,587
Weighted Average Occupancy (as a percent)	94.50%
Manhattan | Retail
Real estate properties
Number of Properties | building	18
Approximate Square Feet (sqft)	756,963
Weighted Average Occupancy (as a percent)	91.20%
Manhattan | Development/Redevelopment
Real estate properties
Number of Properties | building	9
Approximate Square Feet (sqft)	2,025,744
Weighted Average Occupancy (as a percent)	53.80%
Number of properties held for sale | property	2
Manhattan | Fee Interest
Real estate properties
Number of Properties | building	2
Approximate Square Feet (sqft)	783,530
Weighted Average Occupancy (as a percent)	100.00%
Manhattan | Residential
Real estate properties
Number of Properties | building	22
Approximate Square Feet (sqft)	2,972,820
Weighted Average Occupancy (as a percent)	94.60%
Manhattan | Consolidated properties
Real estate properties
Number of Properties | building	42
Approximate Square Feet (sqft)	21,965,576
Manhattan | Consolidated properties | Office
Real estate properties
Number of Properties | building	26
Approximate Square Feet (sqft)	20,700,091
Manhattan | Consolidated properties | Retail
Real estate properties
Number of Properties | building	9
Approximate Square Feet (sqft)	408,993
Manhattan | Consolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	5
Approximate Square Feet (sqft)	72,962
Manhattan | Consolidated properties | Fee Interest
Real estate properties
Number of Properties | building	2
Approximate Square Feet (sqft)	783,530
Manhattan | Consolidated properties | Residential
Real estate properties
Number of Properties | building	4
Approximate Square Feet (sqft)	762,587
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 | building	19
Approximate Square Feet (sqft)	5,629,248
Manhattan | Unconsolidated properties | Office
Real estate properties
Number of Properties | building	6
Approximate Square Feet (sqft)	3,328,496
Manhattan | Unconsolidated properties | Retail
Real estate properties
Number of Properties | building	9
Approximate Square Feet (sqft)	347,970
Manhattan | Unconsolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	4
Approximate Square Feet (sqft)	1,952,782
Manhattan | Unconsolidated properties | Fee Interest
Real estate properties
Number of Properties | building	0
Approximate Square Feet (sqft)	0
Manhattan | Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	18
Approximate Square Feet (sqft)	2,210,233
Suburban
Real estate properties
Number of Properties | building	34
Approximate Square Feet (sqft)	5,209,041
Weighted Average Occupancy (as a percent)	80.30%
Suburban | Office
Real estate properties
Number of Properties | building	31
Approximate Square Feet (sqft)	5,156,041
Weighted Average Occupancy (as a percent)	80.10%
Suburban | Retail
Real estate properties
Number of Properties | building	1
Approximate Square Feet (sqft)	52,000
Weighted Average Occupancy (as a percent)	100.00%
Number of properties held for sale | property	2
Suburban | Development/Redevelopment
Real estate properties
Number of Properties | building	2
Approximate Square Feet (sqft)	1,000
Weighted Average Occupancy (as a percent)	100.00%
Suburban | Residential
Real estate properties
Number of Properties | building	1
Approximate Square Feet (sqft)	66,611
Weighted Average Occupancy (as a percent)	95.80%
Suburban | Consolidated properties
Real estate properties
Number of Properties | building	30
Approximate Square Feet (sqft)	4,503,400
Suburban | Consolidated properties | Office
Real estate properties
Number of Properties | building	28
Approximate Square Feet (sqft)	4,450,400
Suburban | Consolidated properties | Retail
Real estate properties
Number of Properties | building	1
Approximate Square Feet (sqft)	52,000
Suburban | Consolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	1
Approximate Square Feet (sqft)	1,000
Suburban | Consolidated properties | Residential
Real estate properties
Number of Properties | building	1
Approximate Square Feet (sqft)	66,611
Suburban | Unconsolidated properties
Real estate properties
Number of Properties | building	4
Approximate Square Feet (sqft)	705,641
Suburban | Unconsolidated properties | Office
Real estate properties
Number of Properties | building	3
Approximate Square Feet (sqft)	705,641
Suburban | Unconsolidated properties | Retail
Real estate properties
Number of Properties | building	0
Approximate Square Feet (sqft)	0
Suburban | Unconsolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	1
Approximate Square Feet (sqft)	0
Suburban | Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	0
Approximate Square Feet (sqft)	0