Investment in Real Estate Inestment in Real Estate - Summary of Industrial Property Dispositions (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
Proceeds from sale of industrial properties | $	[1]	$ 236,059	$ 169,911	$ 158,429
Gain on sale of real estate | $	[1]	$ 131,269	$ 68,202	$ 48,906
Disposition Activity
Real Estate Properties [Line Items]
Number of industrial properties | Property		60	63	66
Gross leasable area (GLA) of industrial properties | ft²		4.6	3.9	3.8

[1] (A) Gross proceeds and gain on sale of real estate includes the sale of several land parcels for the years ended December 31, 2017 and 2015.