Organization and Basis of Presentation (Schedule of Commercial Office Properties)(Details) $ in Billions	6 Months Ended
	Jun. 30, 2016 USD ($) ft² building shares
Real estate properties
Number of Properties | building	109
Approximate Square Feet unaudited (sqft)	33,018,905
Weighted Average Occupancy unaudited (as a percent)	88.80%
Debt and preferred equity investments including investments held by unconsolidated joint ventures | $	$ 1.7
Debt and preferred equity investments and other financing receivables included in other balance sheet items | $	$ 0.3
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	88
Approximate Square Feet unaudited (sqft)	29,299,036
Weighted Average Occupancy unaudited (as a percent)	91.50%
Residential
Real estate properties
Number of Properties | building	21
Approximate Square Feet unaudited (sqft)	3,719,869
Weighted Average Occupancy unaudited (as a percent)	67.50%
Managed office properties
Real estate properties
Approximate Square Feet unaudited (sqft)	336,000
Consolidated properties
Real estate properties
Number of Properties | building	71
Approximate Square Feet unaudited (sqft)	24,057,221
Consolidated properties | Commercial properties
Real estate properties
Number of Properties | building	67
Approximate Square Feet unaudited (sqft)	23,294,634
Consolidated properties | Residential
Real estate properties
Number of Properties | building	4
Approximate Square Feet unaudited (sqft)	762,587
Unconsolidated properties
Real estate properties
Number of Properties | building	38
Approximate Square Feet unaudited (sqft)	8,961,684
Unconsolidated properties | Commercial properties
Real estate properties
Number of Properties | building	21
Approximate Square Feet unaudited (sqft)	6,004,402
Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	17
Approximate Square Feet unaudited (sqft)	2,957,282
Manhattan
Real estate properties
Number of Properties | building	57
Approximate Square Feet unaudited (sqft)	24,370,736
Weighted Average Occupancy unaudited (as a percent)	93.40%
Manhattan | Office
Real estate properties
Number of Properties | building	31
Approximate Square Feet unaudited (sqft)	22,612,745
Weighted Average Occupancy unaudited (as a percent)	95.60%
Manhattan | Retail
Real estate properties
Number of Properties | building	19
Approximate Square Feet unaudited (sqft)	771,515
Weighted Average Occupancy unaudited (as a percent)	87.80%
Manhattan | Development/Redevelopment
Real estate properties
Number of Properties | building	6
Approximate Square Feet unaudited (sqft)	809,946
Weighted Average Occupancy unaudited (as a percent)	34.90%
Manhattan | Fee Interest
Real estate properties
Number of Properties | building	1
Approximate Square Feet unaudited (sqft)	176,530
Weighted Average Occupancy unaudited (as a percent)	100.00%
Manhattan | Residential
Real estate properties
Number of Properties | building	21
Approximate Square Feet unaudited (sqft)	3,719,869
Weighted Average Occupancy unaudited (as a percent)	67.50%
Manhattan | Consolidated properties
Real estate properties
Number of Properties | building	39
Approximate Square Feet unaudited (sqft)	19,006,334
Manhattan | Consolidated properties | Office
Real estate properties
Number of Properties | building	25
Approximate Square Feet unaudited (sqft)	18,368,606
Manhattan | Consolidated properties | Retail
Real estate properties
Number of Properties | building	10
Approximate Square Feet unaudited (sqft)	418,563
Manhattan | Consolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	3
Approximate Square Feet unaudited (sqft)	42,635
Manhattan | Consolidated properties | Fee Interest
Real estate properties
Number of Properties | building	1
Approximate Square Feet unaudited (sqft)	176,530
Manhattan | Consolidated properties | Residential
Real estate properties
Number of Properties | building	4
Approximate Square Feet unaudited (sqft)	762,587
Manhattan | Consolidated properties | Dual property type, retail portion
Real estate properties
Approximate Square Feet unaudited (sqft)	270,132
Manhattan | Consolidated properties | Dual property type, residential portion
Real estate properties
Approximate Square Feet unaudited (sqft)	222,855
Manhattan | Unconsolidated properties
Real estate properties
Number of Properties | building	18
Approximate Square Feet unaudited (sqft)	5,364,402
Manhattan | Unconsolidated properties | Office
Real estate properties
Number of Properties | building	6
Approximate Square Feet unaudited (sqft)	4,244,139
Manhattan | Unconsolidated properties | Retail
Real estate properties
Number of Properties | building	9
Approximate Square Feet unaudited (sqft)	352,952
Manhattan | Unconsolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	3
Approximate Square Feet unaudited (sqft)	767,311
Manhattan | Unconsolidated properties | Fee Interest
Real estate properties
Number of Properties | building	0
Approximate Square Feet unaudited (sqft)	0
Manhattan | Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	17
Approximate Square Feet unaudited (sqft)	2,957,282
Suburban
Real estate properties
Number of Properties | building	31
Approximate Square Feet unaudited (sqft)	4,928,300
Weighted Average Occupancy unaudited (as a percent)	82.30%
Suburban | Office
Real estate properties
Number of Properties | building	28
Approximate Square Feet unaudited (sqft)	4,875,300
Weighted Average Occupancy unaudited (as a percent)	82.10%
Suburban | Retail
Real estate properties
Number of Properties | building	1
Approximate Square Feet unaudited (sqft)	52,000
Weighted Average Occupancy unaudited (as a percent)	100.00%
Suburban | Development/Redevelopment
Real estate properties
Number of Properties | building	2
Approximate Square Feet unaudited (sqft)	1,000
Weighted Average Occupancy unaudited (as a percent)	100.00%
Suburban | Residential
Real estate properties
Number of Properties | building	0
Approximate Square Feet unaudited (sqft)	0
Weighted Average Occupancy unaudited (as a percent)	0.00%
Suburban | Consolidated properties
Real estate properties
Number of Properties | building	28
Approximate Square Feet unaudited (sqft)	4,288,300
Suburban | Consolidated properties | Office
Real estate properties
Number of Properties | building	26
Approximate Square Feet unaudited (sqft)	4,235,300
Suburban | Consolidated properties | Retail
Real estate properties
Number of Properties | building	1
Approximate Square Feet unaudited (sqft)	52,000
Suburban | Consolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	1
Approximate Square Feet unaudited (sqft)	1,000
Suburban | Consolidated properties | Residential
Real estate properties
Number of Properties | building	0
Approximate Square Feet unaudited (sqft)	0
Suburban | Unconsolidated properties
Real estate properties
Number of Properties | building	3
Approximate Square Feet unaudited (sqft)	640,000
Suburban | Unconsolidated properties | Office
Real estate properties
Number of Properties | building	2
Approximate Square Feet unaudited (sqft)	640,000
Suburban | Unconsolidated properties | Retail
Real estate properties
Number of Properties | building	0
Approximate Square Feet unaudited (sqft)	0
Suburban | Unconsolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	1
Approximate Square Feet unaudited (sqft)	0
Suburban | Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	0
Approximate Square Feet unaudited (sqft)	0