Commitments and Contingencies - Other (Details) $ in Thousands, ft² in Millions	Jun. 30, 2019 USD ($) ft² properties bldg investment	Dec. 31, 2018 USD ($)	Jun. 30, 2014 ft² investment
Loss Contingencies [Line Items]
Letters of credit outstanding	$ 8,300
Development in progress	416,035	$ 462,572
Acres of developable land | Land Purchases
Loss Contingencies [Line Items]
Remaining amount committed to future acquisitions	44,500
Supply Commitment | Leasehold Improvements
Loss Contingencies [Line Items]
Remaining maximum amount committed for tenant improvements	$ 16,900
Supply Commitment | Construction in Progress
Loss Contingencies [Line Items]
Leaseable square feet (in sqft) | ft²	6.8
Number of buildings (in buildings) | bldg	24
Total anticipated development cost	$ 584,300
Development in progress	416,000
Deferred leasing costs in development	18,000
Remaining amount committed or obligated to pay	$ 1,000
Supply Commitment | Construction in Progress | Equity Method Investments
Loss Contingencies [Line Items]
Number of buildings (in buildings) | properties	3
Supply Commitment | Land, Buildings and Improvements
Loss Contingencies [Line Items]
Remaining amount committed or obligated to pay	$ 12,000
Philadelphia | Liberty Property 18th & Arch
Loss Contingencies [Line Items]
Number of equity method investments (in investments) | investment	2		2
Total anticipated development cost	$ 968,000
Philadelphia | Liberty Property 18th & Arch | Construction in Progress
Loss Contingencies [Line Items]
Leaseable square feet (in sqft) | ft²			1.3
American Water Works, Inc | Supply Commitment | Building and Building Improvements
Loss Contingencies [Line Items]
Other commitment	$ 247,800