Condensed consolidated statement of changes in equity (deficit) - 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, 2018							83,584		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 (loss) income	101,158			88,030	13,128
Equity-based compensation expense	3,565		3,565
Exchanges of Class B common stock (in shares)							886		(886)
Exchanges of Class B common stock			(1,172)		1,172
Exercise of stock options, vesting of restricted share units and ESPP share purchase (in shares)							99
Exercise of stock options, vesting of restricted share units and ESPP share purchase	2,035		2,035
Repurchase and retirement of common stock (in shares)							(2,796)
Repurchase and retirement of common stock	(157,946)		273	(157,945)	(273)		$ (1)
Tax benefit arrangement liability and deferred taxes arising from exchanges of Class B common stock	2,807		2,807
Non-cash adjustments to VIEs	(3,836)				(3,836)
Distributions paid to members of Pla-Fit Holdings	(5,499)				(5,499)
Cumulative effect adjustment	(1,713)			(1,713)
Forfeiture of dividend equivalents	6			6
Other comprehensive income (loss)	95	95
Ending balance (in shares) at Sep. 30, 2019							81,773		8,562
Ending balance at Sep. 30, 2019	(442,117)	189	27,240	(466,032)	(3,523)		$ 8		$ 1
Beginning balance (in shares) at Jun. 30, 2019							83,995		8,582
Beginning balance at Jun. 30, 2019	(314,372)	251	24,495	(333,870)	(5,257)		$ 8		$ 1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	29,692			25,777	3,915
Equity-based compensation expense	1,286		1,286
Exchanges of Class B common stock (in shares)							20		(20)
Exercise of stock options, vesting of restricted share units and ESPP share purchase (in shares)							30
Exercise of stock options, vesting of restricted share units and ESPP share purchase	826		826
Repurchase and retirement of common stock (in shares)							(2,272)
Repurchase and retirement of common stock	(157,945)		209	(157,945)	(209)
Tax benefit arrangement liability and deferred taxes arising from exchanges of Class B common stock	424		424
Non-cash adjustments to VIEs	(215)				(215)
Distributions paid to members of Pla-Fit Holdings	(1,757)				(1,757)
Forfeiture of dividend equivalents	6			6
Other comprehensive income (loss)	(62)	(62)
Ending balance (in shares) at Sep. 30, 2019							81,773		8,562
Ending balance at Sep. 30, 2019	(442,117)	189	27,240	(466,032)	(3,523)		$ 8		$ 1
Beginning balance (in shares) at Dec. 31, 2019						78,525	78,525	8,562	8,562
Beginning balance at Dec. 31, 2019	(707,754)	303	29,820	(736,587)	(1,299)		$ 8		$ 1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	(24,886)			(23,681)	(1,205)
Equity-based compensation expense	3,564		3,564
Exchanges of Class B common stock (in shares)							2,518		(2,518)
Exchanges of Class B common stock			(1,526)		1,526
Exercise of stock options, vesting of restricted share units and ESPP share purchase (in shares)							97
Exercise of stock options, vesting of restricted share units and ESPP share purchase	2,153		2,153
Repurchase and retirement of common stock (in shares)							(667)
Repurchase and retirement of common stock	0
Tax benefit arrangement liability and deferred taxes arising from exchanges of Class B common stock	6,660		6,660
Non-cash adjustments to VIEs	(657)				(657)
Distributions paid to members of Pla-Fit Holdings	(1,658)				(1,658)
Other comprehensive income (loss)	(341)	(341)
Ending balance (in shares) at Sep. 30, 2020						80,473	80,473	6,044	6,044
Ending balance at Sep. 30, 2020	(722,919)	(38)	40,671	(760,268)	(3,293)		$ 8		$ 1
Beginning balance (in shares) at Jun. 30, 2020							79,994		6,500
Beginning balance at Jun. 30, 2020	(721,700)	(57)	38,900	(757,139)	(3,413)		$ 8		$ 1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	(3,284)			(3,111)	(173)
Equity-based compensation expense	1,071		1,071
Exchanges of Class B common stock (in shares)							456		(456)
Exchanges of Class B common stock			(570)		570
Exercise of stock options, vesting of restricted share units and ESPP share purchase (in shares)							23
Exercise of stock options, vesting of restricted share units and ESPP share purchase	800		800
Tax benefit arrangement liability and deferred taxes arising from exchanges of Class B common stock	470		470
Non-cash adjustments to VIEs	(219)				(219)
Distributions paid to members of Pla-Fit Holdings	(58)				(58)
Other	(18)			(18)
Other comprehensive income (loss)	19	19
Ending balance (in shares) at Sep. 30, 2020						80,473	80,473	6,044	6,044
Ending balance at Sep. 30, 2020	$ (722,919)	$ (38)	$ 40,671	$ (760,268)	$ (3,293)		$ 8		$ 1