Organization and Description of Business (Narrative) (Details) - property	Jun. 30, 2018	Dec. 31, 2017
Organization and Description of Business [Line Items]
Number of properties	198	205
Common Interest
Organization and Description of Business [Line Items]
Ownership percentage in the Operating Partnership	96.00%	96.00%
Preferred Interest
Organization and Description of Business [Line Items]
Ownership percentage in the Operating Partnership	100.00%	100.00%
United States
Organization and Description of Business [Line Items]
Number of properties	146	152
Europe
Organization and Description of Business [Line Items]
Number of properties	37	38
Asia
Organization and Description of Business [Line Items]
Number of properties	7	7
Australia
Organization and Description of Business [Line Items]
Number of properties	5	5
Canada
Organization and Description of Business [Line Items]
Number of properties	3	3
Operating Properties
Organization and Description of Business [Line Items]
Number of properties	180	180
Operating Properties | United States
Organization and Description of Business [Line Items]
Number of properties	132	131
Operating Properties | Europe
Organization and Description of Business [Line Items]
Number of properties	37	38
Operating Properties | Asia
Organization and Description of Business [Line Items]
Number of properties	3	3
Operating Properties | Australia
Organization and Description of Business [Line Items]
Number of properties	5	5
Operating Properties | Canada
Organization and Description of Business [Line Items]
Number of properties	3	3
Held-for-Sale Properties
Organization and Description of Business [Line Items]
Number of properties	0	7
Held-for-Sale Properties | United States
Organization and Description of Business [Line Items]
Number of properties	0	7
Held-for-Sale Properties | Europe
Organization and Description of Business [Line Items]
Number of properties	0	0
Held-for-Sale Properties | Asia
Organization and Description of Business [Line Items]
Number of properties	0	0
Held-for-Sale Properties | Australia
Organization and Description of Business [Line Items]
Number of properties	0	0
Held-for-Sale Properties | Canada
Organization and Description of Business [Line Items]
Number of properties	0	0
Unconsolidated Joint Ventures
Organization and Description of Business [Line Items]
Number of properties	18	18
Unconsolidated Joint Ventures | United States
Organization and Description of Business [Line Items]
Number of properties	14	14
Unconsolidated Joint Ventures | Europe
Organization and Description of Business [Line Items]
Number of properties	0	0
Unconsolidated Joint Ventures | Asia
Organization and Description of Business [Line Items]
Number of properties	4	4
Unconsolidated Joint Ventures | Australia
Organization and Description of Business [Line Items]
Number of properties	0	0
Unconsolidated Joint Ventures | Canada
Organization and Description of Business [Line Items]
Number of properties	0	0