Business Organization - Additional Information (Detail) member in Millions	9 Months Ended
	Sep. 30, 2019 member store state segment	Jun. 30, 2019 store	Dec. 31, 2018 store	Sep. 30, 2018 store	Jun. 30, 2018 store	Dec. 31, 2017 store	Aug. 05, 2015
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Number of owned and franchised locations | store	1,899	1,859	1,742	1,646	1,608	1,518
Number of states in which entity operates | state	50
Number of reportable segments | segment	3
Pla-Fit Holdings, LLC
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Percentage of ownership	100.00%						100.00%
Percentage of economic interest	90.50%
Pla-Fit Holdings, LLC | Holdings Units
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Percentage of economic interest	9.50%
Planet Intermediate, LLC | Pla-Fit Holdings, LLC
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Percentage of ownership							100.00%
Planet Fitness Holdings, LLC | Planet Intermediate, LLC
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Percentage of ownership							100.00%
Minimum
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Number of members (more than) | member	14.1