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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator
Net income	$ 39,827	$ 30,418	$ 71,466	$ 53,912
Less: net income attributable to non-controlling interests	4,983	4,544	9,213	8,157
Net income attributable to Planet Fitness, Inc.	$ 34,844	$ 25,874	$ 62,253	$ 45,755
Stock Options
Effect of dilutive securities:
Weighted-average shares outstanding adjustment (in shares)	643,120	389,994	614,050	351,987
Restricted Stock Units
Effect of dilutive securities:
Weighted-average shares outstanding adjustment (in shares)	49,088	21,960	49,408	14,886
Class A Common Stock
Denominator
Weighted-average shares of Class A common stock outstanding - basic (in shares)	84,142,975	87,693,377	83,975,192	87,564,596
Effect of dilutive securities:
Weighted-average shares of Class A common stock outstanding - diluted (in shares)	84,835,183	88,105,331	84,638,650	87,931,469
Earnings per share of Class A common stock - basic (in dollars per share)	$ 0.41	$ 0.30	$ 0.74	$ 0.52
Earnings per share of Class A common stock - diluted (in dollars per share)	$ 0.41	$ 0.29	$ 0.74	$ 0.52