Rental Property - Disposals (Details) ft² in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2017 USD ($)	Jun. 30, 2017 USD ($) ft² property building	Dec. 31, 2016 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area (in square feet) | ft²		67.7
Carrying value of property sold		$ 2,433,656,000	$ 2,116,836,000
Disposal Group, Disposed of by Sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Area (in square feet) | ft²		0.2
Number of buildings disposed | building		4
Carrying value of property sold		$ 8,600,000
Net proceeds from sales of rental property		10,309,000
Gain (Loss) on Sale		1,662,000
Contribution to revenue	$ 15,000
Net income (loss) before gain on sale of rental property		$ 100,000
Disposal Group, Held-for-sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties held-for-sale | property		2