Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
	Feb. 19, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Feb. 15, 2019
Subsequent Events (Details) [Line Items]
Dividends per share (in dollars per share)		$ 0.78	$ 0.75	$ 0.72
Subsequent Event
Subsequent Events (Details) [Line Items]
Dividends per share (in dollars per share)	$ 0.1970
Vancouver Market Center | Vancouver, Washington [Member] | Subsequent Event
Subsequent Events (Details) [Line Items]
Sales Price of Property Sold					$ 17.0