Revenue recognition - Schedule of Deferred Revenue (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred Revenue Arrangement [Line Items]
Total deferred revenue	$ 49,862	$ 27,523
Long-term portion of deferred revenue	26,374	8,440
Current portion of deferred revenue	23,488	19,083
Prepaid membership fees
Deferred Revenue Arrangement [Line Items]
Total deferred revenue	6,085	5,198
Enrollment fees
Deferred Revenue Arrangement [Line Items]
Total deferred revenue	1,104	1,014
Equipment discount
Deferred Revenue Arrangement [Line Items]
Total deferred revenue	3,855	2,567
Annual membership fees
Deferred Revenue Arrangement [Line Items]
Total deferred revenue	10,142	8,113
Area development and franchise fees
Deferred Revenue Arrangement [Line Items]
Total deferred revenue	$ 28,676	$ 10,631