Subsequent Events (Details) $ / shares in Units, $ in Millions		1 Months Ended	3 Months Ended				9 Months Ended
	Nov. 02, 2017 $ / shares	Nov. 07, 2017 USD ($) ft² real_estate_property	Sep. 30, 2017 USD ($) real_estate_property $ / shares	Jun. 30, 2017 USD ($) ft² real_estate_property	Mar. 31, 2017 USD ($) ft² real_estate_property	Sep. 30, 2016 $ / shares	Sep. 30, 2017 $ / shares	Sep. 30, 2016 $ / shares
Subsequent Event [Line Items]
Dividend declared (in dollars per share) | $ / shares			$ 0.3925			$ 0.3825	$ 1.17	$ 1.14
Number of Real Estate Properties Acquired | real_estate_property			2	10	10
Area of Real Estate Property | ft²				203,000	145,000
Business Combination, Consideration Transferred | $			$ 28.3	$ 36.2	$ 28.5
Percentage of Acquired Building Leased at Acquisition			100.00%	100.00%	95.20%
Subsequent Event [Member]
Subsequent Event [Line Items]
Dividend declared (in dollars per share) | $ / shares	$ 0.395
Number of Real Estate Properties Acquired | real_estate_property		3
Scenario, Forecast [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Area of Real Estate Property | ft²		105,500
Business Combination, Consideration Transferred | $		$ 25.9
Percentage of Acquired Building Leased at Acquisition		100.00%