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								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator
Net income	$ 28,779	$ 20,472	$ 30,418	$ 23,493	$ 829	$ 18,902	$ 18,004	$ 17,866	$ 103,162	$ 55,601	$ 71,247
Less: net income attributable to non-controlling interests									15,141	22,455	49,747
Net income attributable to Planet Fitness, Inc.	$ 24,796	$ 17,471	$ 25,874	$ 19,880	$ (3,453)	$ 15,345	$ 12,412	$ 8,842	$ 88,021	$ 33,146	$ 21,500
Stock Options
Effect of dilutive securities:
Weighted-average shares outstanding adjustment									417,264	56,198	1,489
Restricted Stock Units
Effect of dilutive securities:
Weighted-average shares outstanding adjustment									22,618	4,962	2,908
Class A Common Stock
Denominator
Weighted-average shares of Class A common stock outstanding - basic (shares)									87,235,021	78,910,390	43,300,288
Effect of dilutive securities:
Weighted-average shares of Class A common stock outstanding - diluted (shares)									87,674,903	78,971,550	43,304,685
Earnings per share of Class A common stock - basic (usd per share)	$ 0.29	$ 0.20	$ 0.30	$ 0.23	$ (0.04)	$ 0.18	$ 0.16	$ 0.14	$ 1.01	$ 0.42	$ 0.50
Earnings per share of Class A common stock - diluted (usd per share)	$ 0.29	$ 0.20	$ 0.29	$ 0.23	$ (0.04)	$ 0.18	$ 0.16	$ 0.14	$ 1.00	$ 0.42	$ 0.50