Impairments (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) property	Jun. 30, 2017 USD ($) property	Jun. 30, 2018 USD ($) property	Jun. 30, 2017 USD ($) property
Asset Impairment Charges [Abstract]
Total provisions for impairment | $	$ 4.0	$ 2.3	$ 18.2	$ 7.7
Number of impaired properties held-for-sale	8		9
Number of impaired properties held-for-investment	1	1	2	4
Number of impaired properties sold	9	9	14	14