Organization and Basis of Presentation - Schedule of Commercial Office Properties (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) ft² building shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Real estate properties
Number of Properties \| building	98
Approximate Square Feet unaudited (sqft)	33,253,213
Mortgage Loans on Real Estate, Commercial and Consumer, Net Including Debt Investments, Other \| $	$ 2,114,041	$ 1,640,412	$ 1,670,020
Weighted Average Occupancy unaudited (as a percent)	91.40%
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	83
Approximate Square Feet unaudited (sqft)	30,124,252
Weighted Average Occupancy unaudited (as a percent)	91.80%
Residential
Real estate properties
Number of Properties \| building	15
Approximate Square Feet unaudited (sqft)	3,128,961
Weighted Average Occupancy unaudited (as a percent)	87.60%
Managed office properties
Real estate properties
Approximate Square Feet unaudited (sqft)	336,000
Mortgage Loans on Real Estate, Commercial and Consumer, Net Including Debt Investments, Other \| $	$ 2,300,000
Debt Investments, Other \| $	$ 200,000
Consolidated properties
Real estate properties
Number of Properties \| building	60
Approximate Square Feet unaudited (sqft)	18,481,009
Consolidated properties \| Commercial properties
Real estate properties
Number of Properties \| building	57
Approximate Square Feet unaudited (sqft)	18,008,904
Consolidated properties \| Residential
Real estate properties
Number of Properties \| building	3
Approximate Square Feet unaudited (sqft)	472,105
Unconsolidated properties
Real estate properties
Number of Properties \| building	38
Approximate Square Feet unaudited (sqft)	14,772,204
Unconsolidated properties \| Commercial properties
Real estate properties
Number of Properties \| building	26
Approximate Square Feet unaudited (sqft)	12,115,348
Unconsolidated properties \| Residential
Real estate properties
Number of Properties \| building	12
Approximate Square Feet unaudited (sqft)	2,656,856
Manhattan
Real estate properties
Number of Properties \| building	58
Approximate Square Feet unaudited (sqft)	26,418,052
Weighted Average Occupancy unaudited (as a percent)	92.90%
Manhattan \| Office
Real estate properties
Number of Properties \| building	32
Approximate Square Feet unaudited (sqft)	24,661,470
Weighted Average Occupancy unaudited (as a percent)	93.90%
Manhattan \| Retail
Real estate properties
Number of Properties \| building	13
Approximate Square Feet unaudited (sqft)	650,553
Weighted Average Occupancy unaudited (as a percent)	94.50%
Manhattan \| Development/Redevelopment
Real estate properties
Number of Properties \| building	11
Approximate Square Feet unaudited (sqft)	929,499
Weighted Average Occupancy unaudited (as a percent)	63.80%
Manhattan \| Fee Interest
Real estate properties
Number of Properties \| building	2
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	15
Approximate Square Feet unaudited (sqft)	3,128,961
Weighted Average Occupancy unaudited (as a percent)	87.60%
Manhattan \| Consolidated properties
Real estate properties
Number of Properties \| building	35
Approximate Square Feet unaudited (sqft)	14,942,704
Manhattan \| Consolidated properties \| Office
Real estate properties
Number of Properties \| building	23
Approximate Square Feet unaudited (sqft)	14,304,606
Manhattan \| Consolidated properties \| Retail
Real estate properties
Number of Properties \| building	4
Approximate Square Feet unaudited (sqft)	302,583
Manhattan \| Consolidated properties \| Development/Redevelopment
Real estate properties
Number of Properties \| building	7
Approximate Square Feet unaudited (sqft)	158,985
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	3
Approximate Square Feet unaudited (sqft)	472,105
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	23
Approximate Square Feet unaudited (sqft)	11,475,348
Manhattan \| Unconsolidated properties \| Office
Real estate properties
Number of Properties \| building	9
Approximate Square Feet unaudited (sqft)	10,356,864
Manhattan \| Unconsolidated properties \| Retail
Real estate properties
Number of Properties \| building	9
Approximate Square Feet unaudited (sqft)	347,970
Manhattan \| Unconsolidated properties \| Development/Redevelopment
Real estate properties
Number of Properties \| building	4
Approximate Square Feet unaudited (sqft)	770,514
Manhattan \| Unconsolidated properties \| Fee Interest
Real estate properties
Number of Properties \| building	1
Approximate Square Feet unaudited (sqft)	0
Manhattan \| Unconsolidated properties \| Residential
Real estate properties
Number of Properties \| building	12
Approximate Square Feet unaudited (sqft)	2,656,856
Suburban
Real estate properties
Number of Properties \| building	25
Approximate Square Feet unaudited (sqft)	3,706,200
Weighted Average Occupancy unaudited (as a percent)	84.30%
Suburban \| Office
Real estate properties
Number of Properties \| building	22
Approximate Square Feet unaudited (sqft)	3,653,200
Weighted Average Occupancy unaudited (as a percent)	84.00%
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	22
Approximate Square Feet unaudited (sqft)	3,066,200
Suburban \| Consolidated properties \| Office
Real estate properties
Number of Properties \| building	20
Approximate Square Feet unaudited (sqft)	3,013,200
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