Commitments and Contingencies (Other) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) ft² bldg	Dec. 31, 2016 USD ($)
Supply Commitment [Line Items]
Letter of credit obligations	$ 5,100
Number of buildings | bldg	37
Development in progress	$ 378,472	$ 267,450
Deferred leasing costs	198,767	$ 179,161
Land Purchases [Member]
Supply Commitment [Line Items]
Remaining amount committed to future land purchases	$ 60,900
Construction in Progress [Member]
Supply Commitment [Line Items]
Number of buildings | bldg	20
Leaseable square feet | ft²	5,600,000
Anticipated aggregate investment	$ 526,700
Deferred leasing costs	13,900
Building and land parcels [Member]
Supply Commitment [Line Items]
Remaining amount committed or obligated to pay for the benefit of current or future tenants	10,500
Tenant Improvements [Member]
Supply Commitment [Line Items]
Maximum tenant improvements not yet completed	18,600
Land and Land Improvements [Member] | Land Purchases [Member]
Supply Commitment [Line Items]
2018 land purchase commitment	32,900
2019 land purchase commitment	$ 28,000
Construction in Progress [Member] | Equity Method Investments [Member] | Construction in Progress [Member]
Supply Commitment [Line Items]
Number of buildings | bldg	3
Anticipated aggregate investment	$ 375,200
Building and Building Improvements [Member] | American Water Works, Inc [Member]
Supply Commitment [Line Items]
Development commitment	176,900
Costs incurred	$ 102,900
Industrial Property [Member] | Building and Building Improvements [Member] | Development commitment [Member]
Supply Commitment [Line Items]
Area of Real Estate Property | ft²	140,000