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Commitments and Contingencies - Supply Commitments (Details)
$ in Thousands, ft² in Millions
9 Months Ended
Sep. 30, 2017
USD ($)
ft²
properties
bldg
Dec. 31, 2016
USD ($)
Supply Commitment [Line Items]    
Development in progress $ 395,089 $ 267,450
Building and Building Improvements [Member] | American Water Works, Inc [Member]    
Supply Commitment [Line Items]    
Fee development commitment 170,100  
Costs incurred to date 71,800  
Supply Commitment [Member] | Construction in Progress [Member]    
Supply Commitment [Line Items]    
Deferred leasing costs in development $ 14,400  
Construction in Progress [Member]    
Supply Commitment [Line Items]    
Leaseable square feet | ft² 5.9  
Number of Real Estate Properties | properties 24  
Total anticipated development cost $ 560,800  
Buildings and land parcels [Member]    
Supply Commitment [Line Items]    
Remaining amount committed or obligated to pay 8,300  
Tenant Improvements [Member]    
Supply Commitment [Line Items]    
Tenant improvements not yet completed $ 26,700  
Equity Method Investments [Member] | Supply Commitment [Member] | Construction in Progress [Member]    
Supply Commitment [Line Items]    
Number of Real Estate Properties | bldg 4  
Remaining amount committed or obligated to pay $ 976,900