Consolidated statement of changes in equity - USD ($) shares in Thousands, $ in Thousands	Total	Accumulated other comprehensive income (loss)	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 at Dec. 31, 2015	$ (1,080)	$ (1,710)	$ 352	$ (14,032)	$ 14,300		$ 4		$ 6
Beginning balance (shares) at Dec. 31, 2015							36,598		62,112
Net income	71,247			21,500	49,747
Equity-based compensation expense	1,728		1,749	(21)
Repurchase and retirement of common stock (shares)									(222)
Repurchase and retirement of common stock	(1,583)		(441)	(1,142)
Exchanges of Class B common stock (shares)							24,705		(24,705)
Exchanges of Class B common stock	0	499	10,976		(11,475)		$ 2		$ (2)
Tax benefit arrangement liability and deferred taxes arising from secondary offerings and other exchanges	21,695		21,695
Exercise of stock options and vesting of restricted share units (shares)							11
Exercise of stock options and vesting of restricted share units	136		136
Dividend paid to holders of Class A common stock	(169,282)			(169,282)
Dividend equivalents paid or payable	(105,628)			(1,085)	(104,543)
Distributions paid to members of Pla-Fit Holdings	(31,838)				(31,838)
Other comprehensive loss	(150)	37			(187)
Ending balance at Dec. 31, 2016	(214,755)	(1,174)	34,467	(164,062)	(83,996)		$ 6		$ 4
Ending balance (shares) at Dec. 31, 2016							61,314		37,185
Net income	55,601			33,146	22,455
Equity-based compensation expense	2,531		2,565	(34)
Repurchase and retirement of common stock (shares)									(150)
Exchanges of Class B common stock (shares)							25,842		(25,842)
Exchanges of Class B common stock	0	(391)	(54,042)		54,433		$ 3		$ (3)
Tax benefit arrangement liability and deferred taxes arising from secondary offerings and other exchanges	28,648		28,648
Exercise of stock options and vesting of restricted share units (shares)							32
Exercise of stock options and vesting of restricted share units	480		480
Dividend paid to holders of Class A common stock	449			32	417
Dividend equivalents paid or payable	(11,060)			(48)	(11,012)
Distributions paid to members of Pla-Fit Holdings	1,169	917			252
Other comprehensive loss	1,169
Ending balance at Dec. 31, 2017	(136,937)	(648)	12,118	(130,966)	(17,451)		$ 9		$ 1
Ending balance (shares) at Dec. 31, 2017						87,188	87,188	11,193	11,193
Net income	103,162			88,021	15,141
Equity-based compensation expense	5,479		5,482	(3)
Repurchase and retirement of common stock (shares)							(5,431)		(9)
Repurchase and retirement of common stock	(342,383)		719	(342,383)	(719)
Exchanges of Class B common stock (shares)							1,736		(1,736)
Exchanges of Class B common stock	0	1	(3,067)		3,066				$ 0
Tax benefit arrangement liability and deferred taxes arising from secondary offerings and other exchanges	3,271		3,271
Exercise of stock options and vesting of restricted share units (shares)							91
Exercise of stock options and vesting of restricted share units	1,209		1,209
Forfeiture of dividend equivalents	113			113
Distributions paid to members of Pla-Fit Holdings	(8,300)				(8,300)
Cumulative effect adjustment (Note 10)	(9,192)			(9,192)
Other comprehensive loss	789	741			48
Ending balance at Dec. 31, 2018	$ (382,789)	$ 94	$ 19,732	$ (394,410)	$ (8,215)		$ 9		$ 1
Ending balance (shares) at Dec. 31, 2018						83,584	83,584	9,448	9,448