Summary of significant accounting policies Other intangible assets (Details)	12 Months Ended
	Dec. 31, 2013
Internet sites
Finite-Lived Intangible Assets [Line Items]
Amortization Period (in years)	10 years
Minimum | Favorable lease intangible assets
Finite-Lived Intangible Assets [Line Items]
Amortization Period (in years)	19 years
Maximum | Favorable lease intangible assets
Finite-Lived Intangible Assets [Line Items]
Amortization Period (in years)	60 years