Revenue - Transaction Price Allocated to the Remaining Performance Obligations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Total revenue	$ 52,642	$ 47,406
Annual Dues And Franchise Sales [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 45,164
Annual dues
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance period	12 months
Revenue, Remaining Performance Obligation	$ 17,031
Total revenue	8,696	$ 8,235
Franchise sales
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 28,133
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-04-01 | Annual Dues And Franchise Sales [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance period	9 months
Revenue, Remaining Performance Obligation	$ 21,927
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-04-01 | Annual dues
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance period	9 months
Revenue, Remaining Performance Obligation	$ 16,029
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-04-01 | Franchise sales
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance period	9 months
Revenue, Remaining Performance Obligation	$ 5,898
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01 | Annual Dues And Franchise Sales [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance period	1 year
Revenue, Remaining Performance Obligation	$ 7,428
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01 | Annual dues
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance period	1 year
Revenue, Remaining Performance Obligation	$ 1,002
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01 | Franchise sales
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance period	1 year
Revenue, Remaining Performance Obligation	$ 6,426
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01 | Annual Dues And Franchise Sales [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance period	1 year
Revenue, Remaining Performance Obligation	$ 5,086
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01 | Franchise sales
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance period	1 year
Revenue, Remaining Performance Obligation	$ 5,086
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01 | Annual Dues And Franchise Sales [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance period	1 year
Revenue, Remaining Performance Obligation	$ 3,653
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01 | Franchise sales
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance period	1 year
Revenue, Remaining Performance Obligation	$ 3,653
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01 | Annual Dues And Franchise Sales [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance period	1 year
Revenue, Remaining Performance Obligation	$ 2,091
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01 | Franchise sales
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance period	1 year
Revenue, Remaining Performance Obligation	$ 2,091
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01 | Annual Dues And Franchise Sales [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance period	1 year
Revenue, Remaining Performance Obligation	$ 952
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01 | Franchise sales
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance period	1 year
Revenue, Remaining Performance Obligation	$ 952
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01 | Annual Dues And Franchise Sales [Member]
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	4,027
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01 | Franchise sales
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 4,027