Gain on Sales of Investment Properties (Details)	12 Months Ended
	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($) item	Dec. 31, 2013 USD ($) item
Sales of investment properties
Sales proceeds		$ 6,918,000	$ 126,785,000
New properties and properties under development
Sales of investment properties
Number of properties sold | item	38	46	75
Sales proceeds	$ 65,800,000	$ 107,200,000	$ 134,200,000
Gain on sales of investment properties	$ 22,200,000	$ 42,100,000	$ 64,700,000
New properties and properties under development | Crest Net Lease, Inc. (Crest)
Sales of investment properties
Number of properties sold | item		1	1
Sales proceeds		$ 820,000	$ 597,000