Condensed consolidated statement of changes in equity (deficit) (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Accumulated other comprehensive (loss) income	Additional paid- in capital	Accumulated deficit	Non-controlling interests	Class A Common Stock	Class A Common Stock Common stock	Class B Common Stock	Class B Common Stock Common stock
Beginning balance (in shares) at Dec. 31, 2017							87,188		11,193
Beginning balance at Dec. 31, 2017	$ (136,937)	$ (648)	$ 12,118	$ (130,966)	$ (17,451)		$ 9		$ 1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	23,493			19,880	3,613
Equity-based compensation expense	998		998
Exchanges of Class B common stock, shares issued							300		(300)
Exchanges of Class B common stock	0	(1)	(673)		674
Exercise of stock options, vesting of restricted share units and ESPP share purchase (in shares)							17
Exercise of stock options, vesting of restricted share units and ESPP share purchase	242		242
Tax benefit arrangement liability and deferred taxes arising from exchanges of Class B common stock	326		326
Forfeiture of dividend equivalents	33			33
Distributions paid to members of Pla-Fit Holdings	(1,734)				(1,734)
Cumulative effect adjustment	(9,192)			(9,192)
Other comprehensive income	337	279			58
Ending balance (in shares) at Mar. 31, 2018							87,505		10,893
Ending balance at Mar. 31, 2018	(122,434)	(370)	13,011	(120,245)	(14,840)		$ 9		$ 1
Beginning balance (in shares) at Dec. 31, 2018						83,584	83,584	9,448	9,448
Beginning balance at Dec. 31, 2018	(382,789)	94	19,732	(394,410)	(8,215)		$ 9		$ 1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	31,639			27,409	4,230
Equity-based compensation expense	1,315		1,315
Exchanges of Class B common stock, shares issued							859		(859)
Exchanges of Class B common stock	0		(1,172)		1,172
Exercise of stock options, vesting of restricted share units and ESPP share purchase (in shares)							20
Exercise of stock options, vesting of restricted share units and ESPP share purchase	505		505
Tax benefit arrangement liability and deferred taxes arising from exchanges of Class B common stock	2,196		2,196
Non-cash adjustments to VIEs	(3,407)				(3,407)
Distributions paid to members of Pla-Fit Holdings	(1,842)				(1,842)
Cumulative effect adjustment	(1,713)			(1,713)
Other comprehensive income	54	54
Ending balance (in shares) at Mar. 31, 2019						84,463	84,463	8,589	8,589
Ending balance at Mar. 31, 2019	$ (354,042)	$ 148	$ 22,576	$ (368,714)	$ (8,062)		$ 9		$ 1