Real Estate Investments - Acquisitions (Details) - Real Estate Acquisition $ in Millions						1 Months Ended	9 Months Ended
	Nov. 01, 2017 USD ($) ft² building	Jul. 31, 2017 USD ($) ft²	Jun. 13, 2017 USD ($) ft²	Jun. 12, 2017 USD ($) ft²	Mar. 06, 2017 USD ($) ft²	Dec. 31, 2017 building	Sep. 30, 2017 USD ($) ft²
Business Acquisition [Line Items]
Purchase price							$ 80.6
Mortgage Note Payable Assumed							(12.1)
Cash consideration							69.5
Real Estate							81.8
Other							$ (0.2)
Square footage | ft²							215,416
Fair value adjustments							$ 0.4
MINNESOTA
Business Acquisition [Line Items]
Date Acquired					Mar. 06, 2017
Purchase price					$ 13.5
Mortgage Note Payable Assumed					0.0
Cash consideration					13.5
Real Estate					13.3
Other					$ 0.2
Square footage | ft²					34,608
CALIFORNIA
Business Acquisition [Line Items]
Date Acquired		Jul. 31, 2017		Jun. 12, 2017
Purchase price		$ 16.3		$ 26.8
Mortgage Note Payable Assumed		0.0		0.0
Cash consideration		16.7		26.8
Real Estate		16.9		26.8
Other		$ (0.2)		$ 0.0
Square footage | ft²		42,780		75,649
DISTRICT OF COLUMBIA
Business Acquisition [Line Items]
Date Acquired			Jun. 13, 2017
Purchase price			$ 24.0
Mortgage Note Payable Assumed			(12.1)
Cash consideration			12.5
Real Estate			24.8
Other			$ (0.2)
Square footage | ft²			62,379
Fair value adjustments							$ 0.4
Subsequent Event | GEORGIA
Business Acquisition [Line Items]
Purchase price	$ 112.1
Square footage | ft²	288,880
Number of properties acquired | building	4
Subsequent Event | WASHINGTON
Business Acquisition [Line Items]
Purchase price	$ 12.7
Square footage | ft²	26,345
Contract to Purchase | GEORGIA
Business Acquisition [Line Items]
Purchase price	$ 193.8
Number of properties acquired | building	8					4