Real estate assets, net - Components of real estate assets (Details) (USD $)	1 Months Ended	6 Months Ended
Jun. 30, 2012	Jun. 30, 2013 property	Dec. 31, 2012
Real Estate [Abstract]
Number of real estate properties directly acquired		6
Real estate directly acquired	$ 0	$ 278,000
Number of real estate properties held for sale		8
Real estate assets held for sale		$ 901,000	$ 0

The cash outflow from the acquisition of a piece of land, anything permanently fixed to it, including buildings, structures on it and so forth; includes real estate intended to generate income for the owner; excludes real estate acquired for use by the owner.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 17

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 13

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3213-108585

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration