Investments in Unconsolidated Joint Ventures Investments in Unconsolidated Joint Ventures (Acquisition, Development and Construction Arrangements/Sale of Joint Venture Interest or Property)(Details) $ in Thousands	1 Months Ended										12 Months Ended
	Sep. 30, 2016 USD ($)	May 31, 2016 USD ($)	Mar. 31, 2016 USD ($)	Feb. 29, 2016 USD ($)	Sep. 30, 2015 USD ($)	Aug. 31, 2015 USD ($)	Sep. 30, 2014 USD ($)	May 31, 2014 USD ($) unit	Mar. 31, 2014 USD ($)	Jan. 31, 2014 USD ($) ft²	Dec. 31, 2016 USD ($) ft²	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Schedule of Equity Method Investments [Line Items]
Acquisition, development and construction arrangements, carrying value											$ 170,164	$ 145,878
Gain (Loss) on Sale											0	14,122	$ 163,059
Employee compensation award											$ 2,300
Approximate Square Feet (sqft) | ft²											32,922,082
West Coast Office portfolio
Schedule of Equity Method Investments [Line Items]
Number of apartment units | unit								1
Participating Financing Due March 2017
Schedule of Equity Method Investments [Line Items]
Acquisition, development and construction arrangements, carrying value											$ 100,000	99,936
Participating Financing Due February 2022
Schedule of Equity Method Investments [Line Items]
Acquisition, development and construction arrangements, carrying value											45,622	45,942
Mezzanine Loan Due July 2036
Schedule of Equity Method Investments [Line Items]
Acquisition, development and construction arrangements, carrying value											$ 24,542	$ 0
EOP Denver
Schedule of Equity Method Investments [Line Items]
Ownership Interest	0.48%
Gross Asset Valuation	$ 180,700
Gain (Loss) on Sale	$ 300
33 Beekman
Schedule of Equity Method Investments [Line Items]
Ownership Interest		45.90%
Gross Asset Valuation		$ 196,000
Gain (Loss) on Sale		33,000
Promote recognized		$ 10,800
EOP Denver
Schedule of Equity Method Investments [Line Items]
Ownership Interest			4.79%
Gross Asset Valuation			$ 180,700
Gain (Loss) on Sale			$ 2,800
7 Renaissance
Schedule of Equity Method Investments [Line Items]
Ownership Interest			50.00%
Gross Asset Valuation			$ 20,700
Gain (Loss) on Sale			$ 4,200
1 Jericho
Schedule of Equity Method Investments [Line Items]
Ownership Interest				66.11%							11.67%	77.78%
Gross Asset Valuation				$ 95,200
Gain (Loss) on Sale				$ 3,300
The Meadows
Schedule of Equity Method Investments [Line Items]
Ownership Interest						50.00%
Gross Asset Valuation						$ 121,100
Gain (Loss) on Sale						$ (1,600)
315 West 36th Street
Schedule of Equity Method Investments [Line Items]
Ownership Interest					35.50%
Gross Asset Valuation					$ 115,000
Gain (Loss) on Sale					$ 16,300
180 Broadway
Schedule of Equity Method Investments [Line Items]
Ownership Interest							25.50%
Gross Asset Valuation							$ 222,500
Gain (Loss) on Sale							16,500
Promote recognized							$ 3,300
747 Madison Avenue
Schedule of Equity Method Investments [Line Items]
Ownership Interest								33.33%
Gross Asset Valuation								$ 160,000
Gain (Loss) on Sale								0
Promote recognized								10,300
Preferred equity investment								30,000
Deferred gain on sale								$ 13,100
West Coast Office portfolio
Schedule of Equity Method Investments [Line Items]
Ownership Interest									42.02%
Gross Asset Valuation									$ 756,000
Gain (Loss) on Sale									$ 85,600
Number of apartment units | unit								5
21-25 West 34th Street
Schedule of Equity Method Investments [Line Items]
Ownership Interest										49.90%
Gross Asset Valuation										$ 114,900
Gain (Loss) on Sale										$ 20,900
Approximate Square Feet (sqft) | ft²										91,300
Joint venture | 747 Madison Avenue | 30 East 65th Street
Schedule of Equity Method Investments [Line Items]
Investment's ownership percentage in joint venture								100.00%