Revenue - Deferred revenue (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Annual dues
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Performance period	12 months
Balance at beginning of period	$ 15,297
New billings	10,430
Revenue recognized	(8,696)
Balance at the end of period	17,031
Revenue recognized	7,500
Franchise sales
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Balance at beginning of period	27,943
New billings	2,534
Revenue recognized	(2,344)
Balance at the end of period	28,133
Revenue recognized	$ 2,100