Revenue recognition - Income Statement (Details) - 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
Revenue:
Total revenue	$ 174,359	$ 136,656	$ 140,550	$ 121,333	$ 134,028	$ 97,496	$ 107,316	$ 91,102	$ 572,898	$ 429,942	$ 378,241
Operating costs and expenses:
Cost of revenue									162,646	129,266	122,317
Store operations									75,005	60,657	60,121
Selling, general and administrative									72,446	60,369	50,008
National advertising fund expense									42,619	0	0
Depreciation and amortization									35,260	31,761	31,502
Other loss (gain)									878	353	(1,369)
Total operating costs and expenses									388,854	282,406	262,579
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
Other expense, net:
Interest income									4,681	54	21
Interest expense									(50,746)	(35,337)	(27,146)
Other (expense) income									(6,175)	316,928	1,371
Total other expense, net									(52,240)	281,645	(25,754)
Income before income taxes									131,804	429,181	89,908
Provision for income taxes									28,642	373,580	18,661
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
Class A Common Stock
Net income attributable to Planet Fitness, Inc.
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
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
Total adjustments | Accounting Standards Update 2014-09
Revenue:
Total revenue									$ (36,528)
Operating costs and expenses:
Cost of revenue									0
Store operations									0
Selling, general and administrative									0
National advertising fund expense									(42,619)
Depreciation and amortization									0
Other loss (gain)									0
Total operating costs and expenses									(42,619)
Income from operations									6,091
Other expense, net:
Interest income									0
Interest expense									0
Other (expense) income									0
Total other expense, net									0
Income before income taxes									6,091
Provision for income taxes									1,437
Net income									4,654
Less net income attributable to non-controlling interests									642
Net income attributable to Planet Fitness, Inc.									4,012
Amounts under Previous Standards
Revenue:
Total revenue									536,370
Operating costs and expenses:
Cost of revenue									162,646
Store operations									75,005
Selling, general and administrative									72,446
National advertising fund expense									0
Depreciation and amortization									35,260
Other loss (gain)									878
Total operating costs and expenses									346,235
Income from operations									190,135
Other expense, net:
Interest income									4,681
Interest expense									(50,746)
Other (expense) income									(6,175)
Total other expense, net									(52,240)
Income before income taxes									137,895
Provision for income taxes									30,079
Net income									107,816
Less net income attributable to non-controlling interests									15,783
Net income attributable to Planet Fitness, Inc.									$ 92,033
Amounts under Previous Standards | Class A Common Stock
Net income attributable to Planet Fitness, Inc.
Basic (usd per share)									$ 1.06
Diluted (usd per share)									$ 1.05
Franchise
Revenue:
Total revenue									$ 175,314	$ 131,983	$ 97,374
Franchise | Total adjustments | Accounting Standards Update 2014-09
Revenue:
Total revenue									5,666
Franchise | Amounts under Previous Standards
Revenue:
Total revenue									180,980
Commission income
Revenue:
Total revenue									6,632	18,172	19,114
Commission income | Total adjustments | Accounting Standards Update 2014-09
Revenue:
Total revenue									0
Commission income | Amounts under Previous Standards
Revenue:
Total revenue									6,632
National advertising fund revenue
Revenue:
Total revenue									42,194	0	0
National advertising fund revenue | Total adjustments | Accounting Standards Update 2014-09
Revenue:
Total revenue									(42,194)
National advertising fund revenue | Amounts under Previous Standards
Revenue:
Total revenue									0
Corporate-owned stores
Revenue:
Total revenue									138,599	112,114	104,721
Corporate-owned stores | Total adjustments | Accounting Standards Update 2014-09
Revenue:
Total revenue									0
Corporate-owned stores | Amounts under Previous Standards
Revenue:
Total revenue									138,599
Equipment
Revenue:
Total revenue									210,159	$ 167,673	$ 157,032
Equipment | Total adjustments | Accounting Standards Update 2014-09
Revenue:
Total revenue									0
Equipment | Amounts under Previous Standards
Revenue:
Total revenue									$ 210,159