Summary of significant accounting policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($) vendor agreement	Dec. 31, 2017 USD ($) vendor	Dec. 31, 2016 USD ($) vendor	Jan. 01, 2019 USD ($)	Jan. 01, 2018 USD ($)	Dec. 31, 2015 USD ($)
Significant Accounting Policies [Line Items]
Restricted cash	$ 30,708,000	$ 0
Stockholders' deficit	(382,789,000)	(136,937,000)	$ (214,755,000)		$ (146,129,000)	$ (1,080,000)
Total deferred revenue	49,862,000	27,523,000
Selling, general and administrative	72,446,000	60,369,000	50,008,000
Impairment charges	$ 0	0
Number of tax receivable agreements | agreement	2
Applicable tax savings (in percentage)	85.00%
Percentage of remaining tax savings (in percentage)	15.00%
Income tax rate maximum tax liability (in percentage)	3.50%
TRA Holders
Significant Accounting Policies [Line Items]
Liability payable under tax benefit obligations	$ 429,233,000
Selling, General and Administrative Expnses
Significant Accounting Policies [Line Items]
Advertising expenses	12,101,000	9,906,000	8,270,000
Placement Services
Significant Accounting Policies [Line Items]
Selling, general and administrative	$ 5,397,000	4,601,000	$ 3,970,000
Individual Franchise Agreements
Significant Accounting Policies [Line Items]
Franchisee initial term	10 years
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Significant Accounting Policies [Line Items]
Restricted cash	$ 0
Stockholders' deficit	14,033,000	$ (9,192,000)
Selling, general and administrative	$ 0
Equipment Purchase | Supplier Concentration Risk
Significant Accounting Policies [Line Items]
Number of vendors | vendor	2	1	2
Equipment Purchase | Supplier Concentration Risk | Vendor one
Significant Accounting Policies [Line Items]
Purchases from vendor	76.00%	91.00%	83.00%
Equipment Purchase | Supplier Concentration Risk | Vendor two
Significant Accounting Policies [Line Items]
Purchases from vendor	13.00%		13.00%
Advertising Purchase | Supplier Concentration Risk
Significant Accounting Policies [Line Items]
Number of vendors | vendor	1	1	2
Advertising Purchase | Supplier Concentration Risk | Vendor one
Significant Accounting Policies [Line Items]
Purchases from vendor	65.00%	63.00%	25.00%
Advertising Purchase | Supplier Concentration Risk | Vendor two
Significant Accounting Policies [Line Items]
Purchases from vendor			16.00%
Maximum
Significant Accounting Policies [Line Items]
Insured amount	$ 250,000,000
Maximum | Area Development Agreements
Significant Accounting Policies [Line Items]
Franchisee initial term	10 years
Maximum | Transfer Fees
Significant Accounting Policies [Line Items]
Total deferred revenue	$ 25,000
Maximum | Accounting Standards Update 2016-02 | Subsequent Event | Scenario, Forecast
Significant Accounting Policies [Line Items]
Operating lease, liability				$ 135,000,000
Operating lease, right-of-use asset				135,000,000
Minimum | Area Development Agreements
Significant Accounting Policies [Line Items]
Franchisee initial term	5 years
Minimum | Transfer Fees
Significant Accounting Policies [Line Items]
Total deferred revenue	$ 10,000
Minimum | Accounting Standards Update 2016-02 | Subsequent Event | Scenario, Forecast
Significant Accounting Policies [Line Items]
Operating lease, liability				125,000,000
Operating lease, right-of-use asset				$ 125,000,000
Planet Fitness NAF, LLC
Significant Accounting Policies [Line Items]
Percentage of franchise membership billing revenue	2.00%