Commitments and Contingencies - Other (Details)	Mar. 31, 2019 USD ($) ft² properties bldg investment	Dec. 31, 2018 USD ($)	Jun. 30, 2014 ft² investment
Loss Contingencies [Line Items]
Letters of credit outstanding	$ 8,300,000
Development in progress	440,593,000	$ 472,169,000
Acres of developable land | Land Purchases
Loss Contingencies [Line Items]
Remaining amount committed to future acquisitions	$ 44,500,000
Building and Building Improvements
Loss Contingencies [Line Items]
Leaseable square feet (in sqft) | ft²	1,069,670
Number of buildings (in buildings) | bldg	5
Supply Commitment | Leasehold Improvements
Loss Contingencies [Line Items]
Remaining maximum amount committed for tenant improvements	$ 19,400,000
Supply Commitment | Construction in Progress
Loss Contingencies [Line Items]
Leaseable square feet (in sqft) | ft²	7,500,000
Number of buildings (in buildings) | bldg	25
Total anticipated development cost	$ 661,100,000
Deferred leasing costs in development	$ 9,100,000
Supply Commitment | Construction in Progress | Equity Method Investments
Loss Contingencies [Line Items]
Number of buildings (in buildings) | properties	2
Remaining amount committed or obligated to pay	$ 564,000
Supply Commitment | Land, Buildings and Improvements
Loss Contingencies [Line Items]
Remaining amount committed or obligated to pay	$ 13,300,000
Philadelphia | Liberty Property 18th & Arch
Loss Contingencies [Line Items]
Number of equity method investments (in investments) | investment	2		2
Total anticipated development cost	$ 962,000,000
Philadelphia | Liberty Property 18th & Arch | Construction in Progress
Loss Contingencies [Line Items]
Leaseable square feet (in sqft) | ft²			1,300,000
Philadelphia | Liberty Property 18th & Arch | Building and Building Improvements
Loss Contingencies [Line Items]
Total anticipated development cost	226,100,000
American Water Works, Inc | Supply Commitment | Building and Building Improvements
Loss Contingencies [Line Items]
Other commitment	$ 236,400,000