Investment in Unconsolidated Joint Ventures (Details) (USD $)					0 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Comcast Innovation and Technology Center [Member] room story sqft	Jun. 30, 2014 Comcast Innovation and Technology Center [Member] investment	Jun. 30, 2014 Comcast corporation lease [Member] Lease Agreements [Member] Comcast Innovation and Technology Center [Member]	Sep. 30, 2014 Comcast corporation lease [Member] Lease Agreements [Member] Comcast Innovation and Technology Center [Member] sqft
Comcast Innovative and Technology Center [Abstract]
Number of equity method investments				2
Number of stories in a high-rise building			59
Net Rentable Area			1,300,000			982,275
Number of hotel rooms			220
Total anticipated development cost			$ 921,000,000
Anticipated investment in joint ventures			184,000,000
Equity Method Investment, Ownership Percentage			20.00%
Real Estate Investments, Joint Ventures	202,570,000	179,655,000	17,400,000
Development in Process	$ 250,506,000	$ 209,187,000	$ 85,500,000
Length of Lease					20 years
Percent of net rentable area						73.40%