Real Estate Properties - Narrative (Details) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) ft² building	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) ft² property building
Real Estate [Abstract]
Real estate improvements | $	$ 43,300	$ 88,500
Number of properties held for sale | property			0
Disposed of by Sale
Real Estate Properties [Line Items]
Number of properties sold | property			30
Number of buildings sold | building	20		62
Square footage (in sqft) | ft²	5,099,423		8,000,000
Gross sales price | $	$ 755,675		$ 1,300,000