Component of Rental Income and Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Leases, Income Statement, Lease Revenue [Abstract]
Recognition of straight-line rents	$ (0.8)	$ (0.8)	$ 2.7
Recognition of amortization of above and below market leases	$ 3.3	$ 3.5	$ 2.9