Investments in Real Estate - Dispositions (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) ft²	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) ft²	Jun. 30, 2016 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable square feet | ft²	1,764,112		1,764,112
Gains on sale of real estate	$ 16,569	$ 11,563	$ 19,237	$ 11,563
Disposal Group, Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable square feet | ft²	563,217		563,217
Contractual Sales Price	$ 65,625		$ 65,625
Gains on sale of real estate	$ 19,237
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Archibald Ave
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable square feet | ft²	62,677		62,677
Contractual Sales Price	$ 6,875		$ 6,875
Gains on sale of real estate	$ 2,668
Disposal Group, Disposed of by Sale, Not Discontinued Operations | 2535 Midway Drive
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable square feet | ft²	373,744		373,744
Contractual Sales Price	$ 40,050		$ 40,050
Gains on sale of real estate	$ 15,974
Disposal Group, Disposed of by Sale, Not Discontinued Operations | 2811 Harbor Boulevard
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Rentable square feet | ft²	126,796		126,796
Contractual Sales Price	$ 18,700		$ 18,700
Gains on sale of real estate	$ 595