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		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator
Net income	$ 29,692	$ 20,472	$ 101,158	$ 74,384
Less: net income attributable to non-controlling interests	3,915	3,001	13,128	11,158
Net income attributable to Planet Fitness, Inc.	$ 25,777	$ 17,471	$ 88,030	$ 63,226
Stock Options
Effect of dilutive securities:
Weighted-average shares outstanding adjustment (in shares)	606,888	382,499	609,788	319,610
Restricted Stock Units
Effect of dilutive securities:
Weighted-average shares outstanding adjustment (in shares)	42,588	27,904	44,775	17,141
Class A Common Stock
Denominator
Weighted-average shares of Class A common stock outstanding - basic (in shares)	83,157,430	88,047,401	83,699,609	87,727,300
Effect of dilutive securities:
Weighted-average shares of Class A common stock outstanding - diluted (in shares)	83,806,906	88,457,804	84,354,172	88,064,051
Earnings per share of Class A common stock - basic (in dollars per share)	$ 0.31	$ 0.20	$ 1.05	$ 0.72
Earnings per share of Class A common stock - diluted (in dollars per share)	$ 0.31	$ 0.20	$ 1.04	$ 0.72