Summary of significant accounting policies (Details)	3 Months Ended
	Mar. 31, 2013
Summary of significant accounting policies [Line Items]
Penalty period for failing REIT tests	4 years
Minimum [Member]
Summary of significant accounting policies [Line Items]
Lease term	1 year
Minimum [Member] | Residential real estate [Member]
Summary of significant accounting policies [Line Items]
Estimated useful lives	3 years
Maximum [Member]
Summary of significant accounting policies [Line Items]
Lease term	2 years
Maximum [Member] | Residential real estate [Member]
Summary of significant accounting policies [Line Items]
Estimated useful lives	27 years 6 months