Commitments and Contingencies - Supply Commitments (Details) (USD $)
Mar. 31, 2015
Dec. 31, 2014
Supply Commitment [Line Items]    
Development in Process $ 269,518,000 $ 277,411,000
Construction in Progress [Member]    
Supply Commitment [Line Items]    
Net Rentable Area 3,800,000  
Number of Real Estate Properties 21  
Total anticipated development cost 412,900,000  
Deferred Costs, Leasing, Gross 8,000,000  
Construction in Progress [Member] | Comcast Innovation and Technology Center [Member]    
Supply Commitment [Line Items]    
Net Rentable Area 1,300,000  
Total anticipated development cost 932,000,000  
Comcast Innovative and Technology Center [Abstract]    
Number of equity method investments 2  
Number of stories in a high-rise building 59  
Number of hotel rooms 222  
Anticipated investment in joint ventures 185,000,000  
Equity Method Investment, Ownership Percentage 20.00%  
Development Not Yet Commenced [Member]    
Supply Commitment [Line Items]    
Total anticipated development cost 167,000,000  
Buildings and land parcels [Member]    
Supply Commitment [Line Items]    
Remaining amount committed or obligated to pay for the benefit of current or future tenants 14,800,000  
Tenant Improvements [Member]    
Supply Commitment [Line Items]    
Tenant improvements not yet completed $ 41,500,000