Organization and Basis of Presentation (Details 2) $ in Thousands	6 Months Ended
	Jun. 30, 2015 USD ($) ft² property building shares	Dec. 31, 2014 USD ($)
Real estate properties
Number of Properties | building	119
Approximate Square Feet (sqft)	34,875,769
Weighted Average Occupancy (as a percent)	89.70%
Carrying Value, Net of Discounts and Deferred Origination Fees | $	$ 1,685,234	$ 1,408,804
Number of shares to be received on redemption of one unit of limited partnership interests (shares) | shares	1
Office
Real estate properties
Number of properties held for sale | property	1
Retail
Real estate properties
Number of properties held for sale | property	2
Commercial properties
Real estate properties
Number of Properties | building	97
Approximate Square Feet (sqft)	31,999,838
Weighted Average Occupancy (as a percent)	89.10%
Residential
Real estate properties
Number of Properties | building	22
Approximate Square Feet (sqft)	2,875,931
Weighted Average Occupancy (as a percent)	96.30%
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)	25,832,770
Consolidated properties | Commercial properties
Real estate properties
Number of Properties | building	72
Approximate Square Feet (sqft)	25,003,572
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	42
Approximate Square Feet (sqft)	9,042,999
Unconsolidated properties | Commercial properties
Real estate properties
Number of Properties | building	25
Approximate Square Feet (sqft)	6,996,266
Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	17
Approximate Square Feet (sqft)	2,046,733
Manhattan
Real estate properties
Number of Properties | building	61
Approximate Square Feet (sqft)	26,208,697
Weighted Average Occupancy (as a percent)	90.60%
Manhattan | Office
Real estate properties
Number of Properties | building	31
Approximate Square Feet (sqft)	22,009,160
Weighted Average Occupancy (as a percent)	96.90%
Manhattan | Retail
Real estate properties
Number of Properties | building	16
Approximate Square Feet (sqft)	683,363
Weighted Average Occupancy (as a percent)	91.90%
Manhattan | Development/Redevelopment
Real estate properties
Number of Properties | building	12
Approximate Square Feet (sqft)	2,732,644
Weighted Average Occupancy (as a percent)	36.90%
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	21
Approximate Square Feet (sqft)	2,809,320
Weighted Average Occupancy (as a percent)	96.40%
Manhattan | Consolidated properties
Real estate properties
Number of Properties | building	42
Approximate Square Feet (sqft)	20,500,172
Manhattan | Consolidated properties | Office
Real estate properties
Number of Properties | building	24
Approximate Square Feet (sqft)	18,533,045
Manhattan | Consolidated properties | Retail
Real estate properties
Number of Properties | building	9
Approximate Square Feet (sqft)	403,735
Manhattan | Consolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	7
Approximate Square Feet (sqft)	779,862
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)	5,708,525
Manhattan | Unconsolidated properties | Office
Real estate properties
Number of Properties | building	7
Approximate Square Feet (sqft)	3,476,115
Manhattan | Unconsolidated properties | Retail
Real estate properties
Number of Properties | building	7
Approximate Square Feet (sqft)	279,628
Manhattan | Unconsolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	5
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	17
Approximate Square Feet (sqft)	2,046,733
Suburban
Real estate properties
Number of Properties | building	36
Approximate Square Feet (sqft)	5,791,141
Weighted Average Occupancy (as a percent)	82.10%
Suburban | Office
Real estate properties
Number of Properties | building	33
Approximate Square Feet (sqft)	5,738,141
Weighted Average Occupancy (as a percent)	81.90%
Suburban | Retail
Real estate properties
Number of Properties | building	1
Approximate Square Feet (sqft)	52,000
Weighted Average Occupancy (as a percent)	100.00%
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)	92.00%
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	6
Approximate Square Feet (sqft)	1,287,741
Suburban | Unconsolidated properties | Office
Real estate properties
Number of Properties | building	5
Approximate Square Feet (sqft)	1,287,741
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