Summary of Significant Accounting Policies and Procedures and Recent Accounting Pronouncements - Provision For Impairment (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) property	Dec. 31, 2016 USD ($) property item	Dec. 31, 2015 USD ($) property
Summary of Significant Accounting Policies and Procedures and Recent Accounting Pronouncements
Number of properties classified as held for sale	9
Total provisions for impairment | $	$ 14,800,000	$ 20,700,000	$ 10,600,000
Number of impaired properties held-for-sale	3
Number of impaired properties held-for-investment	5	4	2
Number of impaired properties sold	18	35	12
Number of impaired properties disposed from other than sale			1
Impairment on existing goodwill | $	$ 0	$ 0	$ 0
Noncontrolling Interests
Number of properties acquired		2
Number of joint ventures controlling interest acquired | item		2