Real Estate Investments - Acquisitions (Details) - Real Estate Acquisition $ in Millions					6 Months Ended
	Jul. 31, 2017 USD ($) ft²	Jun. 13, 2017 USD ($) ft²	Jun. 12, 2017 USD ($) ft²	Mar. 06, 2017 USD ($) ft²	Jun. 30, 2017 USD ($) ft²
Business Acquisition [Line Items]
Purchase price					$ 64.3
Mortgage Note Payable Assumed					(12.1)
Cash consideration					52.8
Real Estate					64.9
Other					$ 0.0
Square footage | ft²					172,636
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			Jun. 12, 2017
Purchase price			$ 26.8
Mortgage Note Payable Assumed			0.0
Cash consideration			26.8
Real Estate			26.8
Other			$ 0.0
Square footage | ft²			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 | CALIFORNIA
Business Acquisition [Line Items]
Purchase price	$ 16.3
Square footage | ft²	42,780