Investment in Unconsolidated Joint Ventures (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2014 bldg	Jun. 30, 2014 bldg
Comcast Innovation and Technology Center [Member]
Schedule of Equity Method Investments [Line Items]
Number of equity method investments	2	2
Number of stories in a high-rise building	59	59
development total cost	$ 933	$ 933
Public Grant award	40	40
Anticipated investment in joint ventures	185	185
Equity Method Investment, Ownership Percentage	20.00%	20.00%
Real Estate Investments, Joint Ventures	14.2	14.2
Net Rentable Area	1,334,000	1,334,000
Number of hotel rooms	222	222
Land, Buildings and Improvements [Member]
Schedule of Equity Method Investments [Line Items]
Number of real estate properties acquired	6	8
Total purchase price	53.2	90.9
Land, Buildings and Improvements [Member] | Liberty Venture I, LP [Member]
Schedule of Equity Method Investments [Line Items]
Number of real estate properties acquired	3	3
Net rentable area acquired	603,000	603,000
Total purchase price	$ 32.2	$ 32.2
Comcast corporation lease [Member] | Lease Agreements [Member] | Comcast Innovation and Technology Center [Member]
Schedule of Equity Method Investments [Line Items]
Length of Lease	20 years
Net Rentable Area	982,275	982,275
Percent of net rentable area	74.00%	74.00%