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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator
Net income	$ 30,418	$ 18,004	$ 53,912	$ 35,870
Less: net income attributable to non-controlling interests	4,544	5,592	8,157	14,616
Net income attributable to Planet Fitness, Inc.	$ 25,874	$ 12,412	$ 45,755	$ 21,254
Stock Options
Effect of dilutive securities:
Weighted-average shares outstanding adjustment (in shares)	389,994	33,564	351,987	28,893
Restricted Stock Units
Effect of dilutive securities:
Weighted-average shares outstanding adjustment (in shares)	21,960	5,708	14,886	5,121
Class A Common Stock
Denominator
Weighted-average shares of Class A common stock outstanding - basic (in shares)	87,693,377	79,153,778	87,564,596	71,678,755
Effect of dilutive securities:
Weighted-average shares of Class A common stock outstanding - diluted (in shares)	88,105,331	79,193,050	87,931,469	71,712,769
Earnings per share of Class A common stock - basic (in dollars per share)	$ 0.30	$ 0.16	$ 0.52	$ 0.30
Earnings per share of Class A common stock - diluted (in dollars per share)	$ 0.29	$ 0.16	$ 0.52	$ 0.30