Investments in Real Estate and Real Estate Under Construction - Summary of forward purchase commitments (Details) ft² in Thousands, $ in Thousands	6 Months Ended
	Jun. 30, 2017 USD ($) ft²
Real Estate [Line Items]
Square Feet (000's) | ft²	760
Maximum Acquisition Cost	$ 86,330
Industrial Property [Member] | Warren, MI [Member]
Real Estate [Line Items]
Square Feet (000's) | ft²	260
Maximum Acquisition Cost	$ 47,000
Approximate Lease Term (Yrs)	15 years
Letters of credit outstanding, amount	$ 4,600
Industrial Property [Member] | Romulus, Michigan [Member]
Real Estate [Line Items]
Square Feet (000's) | ft²	500
Maximum Acquisition Cost	$ 39,330
Approximate Lease Term (Yrs)	15 years