Impairments (Details)	3 Months Ended	9 Months Ended
Sep. 30, 2017 USD ($) property	Sep. 30, 2016 USD ($) property	Sep. 30, 2017 USD ($) property	Sep. 30, 2016 USD ($) property
Impairments
Total provisions for impairment \| $	$ 365,000	$ 8,800,000	$ 8,100,000	$ 17,000,000
Number of impaired properties sold	3	15	10	29
Number of impaired properties held-for-sale		1	1	1
Number of impaired properties held-for-investment		2	6	2

The aggregate amount of write-downs for impairments recognized during the period for long-lived assets held for abandonment, exchange or sale.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=68055307&loc=d3e2611-110228

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Name:	us-gaap_ImpairmentOfLongLivedAssetsToBeDisposedOf
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration