Revenue recognition - Income Statement (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue:
Franchise	$ 45,417	$ 32,791	$ 87,579	$ 63,072
Commission income	1,575	5,003	3,563	11,519
National advertising fund revenue	11,158	0	21,620	0
Corporate-owned stores	34,252	28,285	66,959	55,326
Equipment	48,148	41,237	82,161	68,501
Total revenue	140,550	107,316	261,882	198,418
Operating costs and expenses:
Cost of revenue	36,744	31,452	63,244	52,576
Store operations	18,047	14,604	36,403	29,788
Selling, general and administrative	17,210	14,768	34,831	28,588
National advertising fund expense	11,158	0	21,620	0
Depreciation and amortization	8,619	7,894	17,084	15,845
Other loss (gain)	(39)	348	971	316
Total operating costs and expenses	91,739	69,066	174,153	127,113
Income from operations	48,811	38,250	87,729	71,305
Other expense, net:
Interest expense, net	(8,628)	(9,028)	(17,361)	(17,791)
Other expense	(502)	(933)	(310)	(251)
Total other expense, net	(9,130)	(9,961)	(17,671)	(18,042)
Income before income taxes	39,681	28,289	70,058	53,263
Provision for income taxes	9,263	10,285	16,146	17,393
Net income	30,418	18,004	53,912	35,870
Less net income attributable to non-controlling interests	4,544	5,592	8,157	14,616
Net income attributable to Planet Fitness, Inc.	$ 25,874	$ 12,412	$ 45,755	$ 21,254
Class A Common Stock
Net income per share of Class A common stock:
Basic (in dollars per share)	$ 0.30	$ 0.16	$ 0.52	$ 0.30
Diluted (in dollars per share)	$ 0.29	$ 0.16	$ 0.52	$ 0.30
Total adjustments | Accounting Standards Update 2014-09
Revenue:
Franchise	$ 762		$ 2,527
Commission income	0		0
National advertising fund revenue	(11,158)		(21,620)
Corporate-owned stores	0		0
Equipment	0		0
Total revenue	(10,396)		(19,093)
Operating costs and expenses:
Cost of revenue	0		0
Store operations	0		0
Selling, general and administrative	0		0
National advertising fund expense	(11,158)		(21,620)
Depreciation and amortization	0		0
Other loss (gain)	0		0
Total operating costs and expenses	(11,158)		(21,620)
Income from operations	762		2,527
Other expense, net:
Interest expense, net	0		0
Other expense	0		0
Total other expense, net	0		0
Income before income taxes	762		2,527
Provision for income taxes	200		624
Net income	562		1,903
Less net income attributable to non-controlling interests	83		279
Net income attributable to Planet Fitness, Inc.	479		1,624
Calculated under Revenue Guidance in Effect before Topic 606
Revenue:
Franchise	46,179		90,106
Commission income	1,575		3,563
National advertising fund revenue	0		0
Corporate-owned stores	34,252		66,959
Equipment	48,148		82,161
Total revenue	130,154		242,789
Operating costs and expenses:
Cost of revenue	36,744		63,244
Store operations	18,047		36,403
Selling, general and administrative	17,210		34,831
National advertising fund expense	0		0
Depreciation and amortization	8,619		17,084
Other loss (gain)	(39)		971
Total operating costs and expenses	80,581		152,533
Income from operations	49,573		90,256
Other expense, net:
Interest expense, net	(8,628)		(17,361)
Other expense	(502)		(310)
Total other expense, net	(9,130)		(17,671)
Income before income taxes	40,443		72,585
Provision for income taxes	9,463		16,770
Net income	30,980		55,815
Less net income attributable to non-controlling interests	4,627		8,436
Net income attributable to Planet Fitness, Inc.	$ 26,353		$ 47,379
Calculated under Revenue Guidance in Effect before Topic 606 | Class A Common Stock
Net income per share of Class A common stock:
Basic (in dollars per share)	$ 0.30		$ 0.54
Diluted (in dollars per share)	$ 0.30		$ 0.54