Stockholder's Equity - Additional Information (Detail) - USD ($)			3 Months Ended		9 Months Ended
	Apr. 30, 2019	Nov. 14, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Nov. 13, 2018	Aug. 03, 2018
Class of Stock [Line Items]
Stock repurchase program, authorized amount								$ 500,000,000
Repurchase and retirement of common stock			$ 157,945,000	$ 42,090,000	$ 157,946,000	$ 42,090,000
Pla-Fit Holdings, LLC
Class of Stock [Line Items]
Number of shares exchanged					885,810
Holdings Units
Class of Stock [Line Items]
Number of shares exchanged					885,810
Investor | Secondary Offering and Exchange
Class of Stock [Line Items]
Number of units held by owners (in shares)			81,773,267		81,773,267
Investor | Pla-Fit Holdings, LLC | Secondary Offering and Exchange
Class of Stock [Line Items]
Percentage of economic interest					90.50%
Continuing LLC Owners | Secondary Offering and Exchange
Class of Stock [Line Items]
Number of units held by owners (in shares)			8,561,920		8,561,920
Continuing LLC Owners | Pla-Fit Holdings, LLC | Secondary Offering and Exchange
Class of Stock [Line Items]
Percentage of economic interest					9.50%
Holdings Units
Class of Stock [Line Items]
Shares exchanged for Class A common stock					1
Class A Common Stock
Class of Stock [Line Items]
Number of shares exchanged					885,810
Class A Common Stock | Investor | Secondary Offering and Exchange
Class of Stock [Line Items]
Number of units held by owners (in shares)			81,773,267		81,773,267
Class A Common Stock | Investor | Pla-Fit Holdings, LLC | Secondary Offering and Exchange
Class of Stock [Line Items]
Percentage of voting interests acquired			90.50%		90.50%
Class B Common Stock
Class of Stock [Line Items]
Shares exchanged for Class A common stock					1
Number of shares exchanged					885,810
Class B Common Stock | Continuing LLC Owners | Secondary Offering and Exchange
Class of Stock [Line Items]
Number of units held by owners (in shares)			8,561,920		8,561,920
Class B Common Stock | Continuing LLC Owners | Pla-Fit Holdings, LLC | Secondary Offering and Exchange
Class of Stock [Line Items]
Percentage of voting interests acquired			9.50%		9.50%
ASR Agreement
Class of Stock [Line Items]
Stock repurchase program, authorized amount							$ 300,000,000
Repurchase and retirement of common stock	$ 60,000,000	$ 240,000,000
ASR Agreement | Class A Common Stock
Class of Stock [Line Items]
Stock repurchased (in shares)	524,124	4,607,410	2,272,001
Weighted average cost per share (in dollars per share)	$ 58.46		$ 69.51