Earnings Per Share - Reconciliation of Numerators and Denominators Used to Compute Basic and Diluted Earnings per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator
Net income	$ 31,639	$ 23,493
Less: net income attributable to non-controlling interests	4,230	3,613
Net income attributable to Planet Fitness, Inc.	$ 27,409	$ 19,880
Stock Options
Effect of dilutive securities:
Weighted-average shares outstanding adjustment (in shares)	569,864	255,527
Restricted Stock Units
Effect of dilutive securities:
Weighted-average shares outstanding adjustment (in shares)	49,866	7,774
Class A Common Stock
Denominator
Weighted-average shares of Class A common stock outstanding - basic (in shares)	83,805,545	87,434,384
Effect of dilutive securities:
Weighted-average shares of Class A common stock outstanding - diluted (in shares)	84,425,275	87,697,685
Earnings per share of Class A common stock - basic (in dollars per share)	$ 0.33	$ 0.23
Earnings per share of Class A common stock - diluted (in dollars per share)	$ 0.32	$ 0.23