Summary of significant accounting policies - Other intangible assets (Details)	12 Months Ended
	Dec. 31, 2017
Minimum | Favorable lease intangible assets
Finite-Lived Intangible Assets
Amortization period (in years)	19 years
Minimum | Internet sites
Finite-Lived Intangible Assets
Amortization period (in years)	5 years
Maximum | Favorable lease intangible assets
Finite-Lived Intangible Assets
Amortization period (in years)	60 years
Maximum | Internet sites
Finite-Lived Intangible Assets
Amortization period (in years)	10 years