Quarterly financial data (Unaudited) - Schedule of Quarterly Financial Data (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
Quarterly Financial Information [Line Items]
Total revenue	$ 174,359	$ 136,656	$ 140,550	$ 121,333	$ 134,028	$ 97,496	$ 107,316	$ 91,102	$ 572,898	$ 429,942	$ 378,241
Income from operations	52,742	43,573	48,811	38,918	42,277	33,954	38,250	33,055	184,044	147,536	115,662
Net income	28,779	20,472	30,418	23,493	829	18,902	18,004	17,866	103,162	55,601	71,247
Net income (loss) 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
Class A Common Stock
Earnings (loss) per share:
Class A - 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
Class A - 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