Other intangible assets - Narratives (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Finite Lived Intangible Assets
Additions			$ 6,100
Amortization expense	$ 135	$ 148	268	$ 271
Estimated amortization expense, remainder of 2017	268		268
Estimated amortization expense, year ending December 31, 2018	536		536
Estimated amortization expense, year ending December 31, 2019	536		536
Estimated amortization expense, year ending December 31, 2020	536		536
Estimated amortization expense, year ending December 31, 2021	536		536
Estimated amortization expense, year ending December 31, 2022	$ 536		536
Trade names
Finite Lived Intangible Assets
Additions			6,100
Favorable lease assets
Finite Lived Intangible Assets
Additions			0
Amortization expense			$ 184
Favorable lease assets | Minimum
Finite Lived Intangible Assets
Amortization period (in years)			19 years
Favorable lease assets | Maximum
Finite Lived Intangible Assets
Amortization period (in years)			60 years
Internet sites
Finite Lived Intangible Assets
Additions			$ 0
Amortization expense			$ 84
Internet sites | Minimum
Finite Lived Intangible Assets
Amortization period (in years)			5 years
Internet sites | Maximum
Finite Lived Intangible Assets
Amortization period (in years)			10 years