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Commitments and Contingencies - Supply Commitments (Details)
$ in Thousands, ft² in Millions
6 Months Ended
Jun. 30, 2017
USD ($)
ft²
bldg
properties
Dec. 31, 2016
USD ($)
Supply Commitment [Line Items]    
Development in progress $ 410,027 $ 267,450
Supply Commitment [Member] | Construction in Progress [Member]    
Supply Commitment [Line Items]    
Deferred leasing costs in development 15,400  
Remaining amount committed or obligated to pay $ 11,800  
Construction in Progress [Member]    
Supply Commitment [Line Items]    
Leaseable square feet | ft² 6.6  
Number of Real Estate Properties | properties 27  
Total anticipated development cost $ 634,000  
Buildings and land parcels [Member]    
Supply Commitment [Line Items]    
Remaining amount committed or obligated to pay 18,600  
Tenant Improvements [Member]    
Supply Commitment [Line Items]    
Tenant improvements not yet completed $ 21,100  
Equity Method Investments [Member] | Supply Commitment [Member] | Construction in Progress [Member]    
Supply Commitment [Line Items]    
Number of Real Estate Properties | bldg 3  
Remaining amount committed or obligated to pay $ 966,700  
American Water Works, Inc [Member] | Building and Building Improvements [Member]    
Supply Commitment [Line Items]    
Fee development commitment 170,100  
Costs incurred to date $ 46,700