Investment in Real Estate Investment in Real Estate - Summary of Industrial Property Acquisitions (Details) $ in Thousands, ft² in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) ft² Property	Dec. 31, 2016 USD ($) ft² Property	Dec. 31, 2015 USD ($) ft² Property
Real Estate Properties [Line Items]
Number of industrial properties | Property	488
Gross leasable area (GLA) of industrial properties | ft²	60.2
Acquisition Activity
Real Estate Properties [Line Items]
Number of industrial properties | Property	8	6	8
Gross leasable area (GLA) of industrial properties | ft²	1.1	0.7	1.9
Purchase price of industrial properties acquired | $	$ 174,209	$ 111,130	$ 169,218