Goodwill and intangible assets - Goodwill Rollforward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Beginning balance	$ 176,981	$ 176,981
Additions	22,532	0
Ending balance	199,513	176,981
Franchise
Goodwill [Roll Forward]
Beginning balance	16,938	16,938
Additions	0	0
Ending balance	16,938	16,938
Corporate-owned stores
Goodwill [Roll Forward]
Beginning balance	67,377	67,377
Additions	22,532	0
Ending balance	89,909	67,377
Equipment revenue
Goodwill [Roll Forward]
Beginning balance	92,666	92,666
Additions	0	0
Ending balance	$ 92,666	$ 92,666