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	74,384			63,226	11,158
Equity-based compensation expense	4,137		4,140	(3)
Exchanges of Class B common stock, shares issued							1,640		(1,640)
Exchanges of Class B common stock		1	(2,913)		2,912
Exercise of stock options, vesting of restricted share units and ESPP share purchase (in shares)							81
Exercise of stock options, vesting of restricted share units and ESPP share purchase	1,106		1,106
Repurchase/Retirement of common stock (in shares)							(824)		(9)
Repurchase/Retirement of common stock	(42,090)			(42,090)
Tax benefit arrangement liability and deferred taxes arising from exchanges of Class B common stock	2,786		2,786
Distributions paid to members of Pla-Fit Holdings	(5,369)				(5,369)
Cumulative effect adjustment	(9,192)			(9,192)
Forfeiture of dividend equivalents	61			61
Other comprehensive income	966	903			63
Ending balance (in shares) at Sep. 30, 2018							88,085		9,544
Ending balance at Sep. 30, 2018	(110,148)	256	17,237	(118,964)	(8,687)		$ 9		$ 1
Beginning balance (in shares) at Jun. 30, 2018							87,932		10,471
Beginning balance at Jun. 30, 2018	(91,248)	(385)	14,744	(94,348)	(11,269)		$ 9		$ 1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	20,472			17,471	3,001
Equity-based compensation expense	1,450		1,450
Exchanges of Class B common stock, shares issued							927		(927)
Exchanges of Class B common stock		1	(1,442)		1,441
Exercise of stock options, vesting of restricted share units and ESPP share purchase (in shares)							50
Exercise of stock options, vesting of restricted share units and ESPP share purchase	706		706
Repurchase/Retirement of common stock (in shares)							(824)
Repurchase/Retirement of common stock	(42,090)			(42,090)
Tax benefit arrangement liability and deferred taxes arising from exchanges of Class B common stock	1,779		1,779
Distributions paid to members of Pla-Fit Holdings	(1,866)				(1,866)
Forfeiture of dividend equivalents	3			3
Other comprehensive income	646	640			6
Ending balance (in shares) at Sep. 30, 2018							88,085		9,544
Ending balance at Sep. 30, 2018	(110,148)	256	17,237	(118,964)	(8,687)		$ 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	101,158			88,030	13,128
Equity-based compensation expense	3,565		3,565
Exchanges of Class B common stock, shares issued							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/Retirement of common stock (in shares)							(2,796)
Repurchase/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	95	95
Ending balance (in shares) at Sep. 30, 2019						81,773	81,773	8,562	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 income	29,692			25,777	3,915
Equity-based compensation expense	1,286		1,286
Exchanges of Class B common stock, shares issued							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/Retirement of common stock (in shares)							(2,272)
Repurchase/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	(62)	(62)
Ending balance (in shares) at Sep. 30, 2019						81,773	81,773	8,562	8,562
Ending balance at Sep. 30, 2019	$ (442,117)	$ 189	$ 27,240	$ (466,032)	$ (3,523)		$ 8		$ 1