Subsequent Events (Details) - 2018 acquisitions [Member] $ in Thousands		9 Months Ended
	Oct. 01, 2018 USD ($) ft²	Sep. 30, 2018 USD ($) ft² bldg
Building and Building Improvements [Member]
Subsequent Event [Line Items]
Number of Real Estate Properties | bldg		7
Leaseable square feet		2,487,083
United Kingdom [Member] | Building and Building Improvements [Member] | Operating Segments [Member]
Subsequent Event [Line Items]
Number of Real Estate Properties | bldg		0
Leaseable square feet		0
United Kingdom [Member] | Building and Building Improvements [Member] | Operating Segments [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Number of Real Estate Properties	0
Leaseable square feet	1,100,000
United Kingdom [Member] | Land, Buildings and Improvements [Member] | Operating Segments [Member]
Subsequent Event [Line Items]
Purchase price | $		$ 4,949
United Kingdom [Member] | Land, Buildings and Improvements [Member] | Operating Segments [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Purchase price | $	$ 144,700