Commitments and Contingencies - Supply Commitments (Details) $ in Thousands, ft² in Millions	Jun. 30, 2015 USD ($) ft² properties	Dec. 31, 2014 USD ($)
Supply Commitment [Line Items]
Development in Process	$ 225,154	$ 277,411
Construction in Progress [Member]
Supply Commitment [Line Items]
Net Rentable Area | ft²	3.0
Number of Real Estate Properties | properties	21
Total anticipated development cost	$ 361,200
Deferred Costs, Leasing, Gross	7,100
Development Not Yet Commenced [Member]
Supply Commitment [Line Items]
Total anticipated development cost	184,600
Buildings and land parcels [Member]
Supply Commitment [Line Items]
Remaining amount committed or obligated to pay for the benefit of current or future tenants	39,300
Tenant Improvements [Member]
Supply Commitment [Line Items]
Tenant improvements not yet completed	$ 34,800