Commitments and Contingencies (Other) (Details) $ in Thousands, ft² in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) ft² bldg	Dec. 31, 2015 USD ($)
Supply Commitment [Line Items]
Letter of credit obligations	$ 6,900
Development in progress	267,450	$ 360,948
Deferred leasing costs	190,849	$ 229,798
Tenant improvements	4,000
Land Purchases [Member]
Supply Commitment [Line Items]
Remaining amount committed to future land purchases	$ 11,300
Construction in Progress [Member]
Supply Commitment [Line Items]
Number of buildings | bldg	25
Leaseable square feet | ft²	5.6
Anticipated aggregate investment	$ 537,100
Deferred leasing costs	7,500
Building and land parcels [Member]
Supply Commitment [Line Items]
Remaining amount committed or obligated to pay for the benefit of current or future tenants	15,000
Tenant Improvements [Member]
Supply Commitment [Line Items]
Maximum tenant improvements not yet completed	$ 31,300
Construction in Progress [Member] | Equity Method Investments [Member] | Construction in Progress [Member]
Supply Commitment [Line Items]
Number of buildings | bldg	2
Anticipated aggregate investment	$ 952,500
Building and Building Improvements [Member] | American Water Works, Inc [Member]
Supply Commitment [Line Items]
Other commitment	132,600
Costs incurred	$ 17,200