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	53,912			45,755	8,157
Equity-based compensation expense	2,687		2,690	(3)
Exchanges of Class B common stock, shares issued							713		(713)
Exchanges of Class B common stock	0		(1,471)		1,471
Repurchase/Retirement of common stock (in shares)									(9)
Exercise of stock options, vesting of restricted share units and ESPP share purchase (in shares)							31
Exercise of stock options, vesting of restricted share units and ESPP share purchase	400		400
Tax benefit arrangement liability and deferred taxes arising from exchanges of Class B common stock	1,007		1,007
Forfeiture of dividend equivalents	58			58
Distributions paid to members of Pla-Fit Holdings	(3,503)				(3,503)
Cumulative effect adjustment	(9,192)			(9,192)
Other comprehensive income	320	263			57
Ending balance (in shares) at Jun. 30, 2018							87,932		10,471
Ending balance at Jun. 30, 2018	(91,248)	(385)	14,744	(94,348)	(11,269)		$ 9		$ 1
Beginning balance (in shares) at Mar. 31, 2018							87,505		10,893
Beginning balance at Mar. 31, 2018	(122,434)	(370)	13,011	(120,245)	(14,840)		$ 9		$ 1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	30,418			25,874	4,544
Equity-based compensation expense	1,689		1,692	(3)
Exchanges of Class B common stock, shares issued							413		(413)
Exchanges of Class B common stock	0	1	(798)		797
Repurchase/Retirement of common stock (in shares)									(9)
Exercise of stock options, vesting of restricted share units and ESPP share purchase (in shares)							14
Exercise of stock options, vesting of restricted share units and ESPP share purchase	158		158
Tax benefit arrangement liability and deferred taxes arising from exchanges of Class B common stock	681		681
Forfeiture of dividend equivalents	26			26
Distributions paid to members of Pla-Fit Holdings	(1,769)				(1,769)
Other comprehensive income	(17)	(16)			(1)
Ending balance (in shares) at Jun. 30, 2018							87,932		10,471
Ending balance at Jun. 30, 2018	(91,248)	(385)	14,744	(94,348)	(11,269)		$ 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	71,466			62,253	9,213
Equity-based compensation expense	2,279		2,279
Exchanges of Class B common stock, shares issued							866		(866)
Exchanges of Class B common stock	0		(1,172)		1,172
Repurchase/Retirement of common stock (in shares)							(524)
Repurchase/Retirement of common stock	(1)		64		(64)		$ (1)
Exercise of stock options, vesting of restricted share units and ESPP share purchase (in shares)							69
Exercise of stock options, vesting of restricted share units and ESPP share purchase	1,209		1,209
Tax benefit arrangement liability and deferred taxes arising from exchanges of Class B common stock	2,383		2,383
Non-cash adjustments to VIEs	(3,621)				(3,621)
Distributions paid to members of Pla-Fit Holdings	(3,742)				(3,742)
Cumulative effect adjustment	(1,713)			(1,713)
Other comprehensive income	157	157
Ending balance (in shares) at Jun. 30, 2019						83,995	83,995	8,582	8,582
Ending balance at Jun. 30, 2019	(314,372)	251	24,495	(333,870)	(5,257)		$ 8		$ 1
Beginning balance (in shares) at Mar. 31, 2019							84,463		8,589
Beginning balance at Mar. 31, 2019	(354,042)	148	22,576	(368,714)	(8,062)		$ 9		$ 1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	39,827			34,844	4,983
Equity-based compensation expense	964		964
Exchanges of Class B common stock, shares issued							7		(7)
Repurchase/Retirement of common stock (in shares)							(524)
Repurchase/Retirement of common stock	(1)		64		(64)		$ (1)
Exercise of stock options, vesting of restricted share units and ESPP share purchase (in shares)							49
Exercise of stock options, vesting of restricted share units and ESPP share purchase	704		704
Tax benefit arrangement liability and deferred taxes arising from exchanges of Class B common stock	187		187
Non-cash adjustments to VIEs	(214)				(214)
Distributions paid to members of Pla-Fit Holdings	(1,900)				(1,900)
Other comprehensive income	103	103
Ending balance (in shares) at Jun. 30, 2019						83,995	83,995	8,582	8,582
Ending balance at Jun. 30, 2019	$ (314,372)	$ 251	$ 24,495	$ (333,870)	$ (5,257)		$ 8		$ 1