Gain on Sales of Real Estate (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($) item
Sales of properties
Sales proceeds			$ 6,918
New properties and properties under development
Sales of properties
Number of properties sold | item	77	38	47
Sales proceeds	$ 90,500	$ 65,800	$ 108,100
Gain on sales of properties	22,000	$ 22,200	$ 42,100
Sale of former corporate headquarters building	$ 8,600