Organization and Basis of Presentation (Details 2) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014 sqft building		Dec. 31, 2013
Real estate properties
Number of properties	101
Square Feet (sqft)	34,299,788
Weighted Average Occupancy (percent)	89.50%	[1]
Debt and preferred equity investments	$ 1,493,725		$ 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	3
Square Feet (sqft)	719,948
Weighted Average Occupancy (percent)	94.00%	[1]
Commercial Property
Real estate properties
Number of properties	98
Square Feet (sqft)	33,579,840
Weighted Average Occupancy (percent)	89.40%	[1]
Managed office properties
Real estate properties
Square Feet (sqft)	708,500
Number of office properties managed (property)	2
Consolidated properties
Real estate properties
Number of properties	74
Square Feet (sqft)	24,852,900
Consolidated properties | Residential
Real estate properties
Number of properties	3
Square Feet (sqft)	719,948
Consolidated properties | Commercial Property
Real estate properties
Number of properties	71
Square Feet (sqft)	24,132,952
Unconsolidated properties
Real estate properties
Number of properties	27
Square Feet (sqft)	9,446,888
Unconsolidated properties | Residential
Real estate properties
Number of properties	0
Square Feet (sqft)	0
Unconsolidated properties | Commercial Property
Real estate properties
Number of properties	27
Square Feet (sqft)	9,446,888
Manhattan
Real estate properties
Number of properties	64
Square Feet (sqft)	27,789,699
Weighted Average Occupancy (percent)	91.30%	[1]
Manhattan | Office
Real estate properties
Number of properties	33
Square Feet (sqft)	23,771,460
Weighted Average Occupancy (percent)	94.30%	[1]
Manhattan | Retail
Real estate properties
Number of properties	15
Square Feet (sqft)	821,567
Weighted Average Occupancy (percent)	93.00%	[1]
Manhattan | Residential
Real estate properties
Number of properties	2
Square Feet (sqft)	653,337
Weighted Average Occupancy (percent)	94.70%	[1]
Manhattan | Development
Real estate properties
Number of properties	14
Square Feet (sqft)	2,235,272
Weighted Average Occupancy (percent)	55.00%	[1]
Manhattan | Fee Interest
Real estate properties
Number of properties	2
Square Feet (sqft)	961,400
Weighted Average Occupancy (percent)	100.00%	[1]
Manhattan | Consolidated properties
Real estate properties
Number of properties	42
Square Feet (sqft)	19,630,552
Manhattan | Consolidated properties | Office
Real estate properties
Number of properties	23
Square Feet (sqft)	17,306,045
Manhattan | Consolidated properties | Retail
Real estate properties
Number of properties	7	[2]
Square Feet (sqft)	389,317
Manhattan | Consolidated properties | Residential
Real estate properties
Number of properties	2	[2]
Square Feet (sqft)	653,337
Manhattan | Consolidated properties | Development
Real estate properties
Number of properties	10
Square Feet (sqft)	973,790
Manhattan | Consolidated properties | Fee Interest
Real estate properties
Number of properties	2
Square Feet (sqft)	961,400
Manhattan | Consolidated properties | Dual Property Type, Residential Portion
Real estate properties
Square Feet (sqft)	222,855
Manhattan | Consolidated properties | Dual Property Type, Retail Portion
Real estate properties
Square Feet (sqft)	270,132
Manhattan | Unconsolidated properties
Real estate properties
Number of properties	22
Square Feet (sqft)	8,159,147
Manhattan | Unconsolidated properties | Office
Real estate properties
Number of properties	10
Square Feet (sqft)	6,465,415
Manhattan | Unconsolidated properties | Retail
Real estate properties
Number of properties	8
Square Feet (sqft)	432,250
Manhattan | Unconsolidated properties | Residential
Real estate properties
Number of properties	0
Square Feet (sqft)	0
Manhattan | Unconsolidated properties | Development
Real estate properties
Number of properties	4
Square Feet (sqft)	1,261,482
Manhattan | Unconsolidated properties | Fee Interest
Real estate properties
Number of properties	0
Square Feet (sqft)	0
Suburban
Real estate properties
Number of properties	34
Square Feet (sqft)	5,790,141
Weighted Average Occupancy (percent)	80.00%	[1]
Suburban | Office
Real estate properties
Number of properties	31
Square Feet (sqft)	5,587,500
Weighted Average Occupancy (percent)	80.90%	[1]
Suburban | Retail
Real estate properties
Number of properties	1
Square Feet (sqft)	52,000
Weighted Average Occupancy (percent)	100.00%	[1]
Suburban | Residential
Real estate properties
Number of properties	1
Square Feet (sqft)	66,611
Weighted Average Occupancy (percent)	87.70%	[1]
Suburban | Development
Real estate properties
Number of properties	2
Square Feet (sqft)	150,641
Weighted Average Occupancy (percent)	40.70%	[1]
Suburban | Consolidated properties
Real estate properties
Number of properties	29
Square Feet (sqft)	4,502,400
Suburban | Consolidated properties | Office
Real estate properties
Number of properties	27
Square Feet (sqft)	4,365,400
Suburban | Consolidated properties | Retail
Real estate properties
Number of properties	1
Square Feet (sqft)	52,000
Suburban | Consolidated properties | Residential
Real estate properties
Number of properties	1
Square Feet (sqft)	66,611
Suburban | Consolidated properties | Development
Real estate properties
Number of properties	1
Square Feet (sqft)	85,000
Suburban | Unconsolidated properties
Real estate properties
Number of properties	5
Square Feet (sqft)	1,287,741
Suburban | Unconsolidated properties | Office
Real estate properties
Number of properties	4
Square Feet (sqft)	1,222,100
Suburban | Unconsolidated properties | Retail
Real estate properties
Number of properties	0
Square Feet (sqft)	0
Suburban | Unconsolidated properties | Residential
Real estate properties
Number of properties	0
Square Feet (sqft)	0
Suburban | Unconsolidated properties | Development
Real estate properties
Number of properties	1
Square Feet (sqft)	65,641

[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 March 31, 2014, we owned a building that was comprised of 270,132 square feet of retail space and 222,855 square feet 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.