Revenue recognition - Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 147,784	$ 113,080	$ 113,080	$ 78,521	$ 40,393
Accounts receivable, net	14,932	37,272	37,272
Due from related parties	0	3,020	3,020
Inventory	3,193	2,692	2,692
Restricted assets – national advertising fund	73	499	499
Deferred expenses – national advertising fund	1,648		0
Prepaid expenses	3,796	3,929	3,929
Other receivables	23,343	9,562	9,562
Other current assets	5,916	6,947	6,947
Total current assets	200,685	177,001	177,001
Property and equipment, net	87,570	83,327	83,327
Intangible assets, net	237,092	235,657	235,657
Goodwill	191,038	176,981	176,981
Deferred income taxes	406,699	411,067	407,782
Other assets, net	1,637	11,717	11,717
Total assets	1,124,721	1,095,750	1,092,465
Current liabilities:
Current maturities of long-term debt	7,185	7,185	7,185
Accounts payable	16,268	28,648	28,648
Accrued expenses	14,715	18,590	18,590
Equipment deposits	9,001	6,498	6,498
Restricted liabilities – national advertising fund	73	490	490
Deferred revenue, current	23,186	18,319	19,083
Payable pursuant to tax benefit arrangements, current	25,578	31,062	31,062
Other current liabilities	436	474	474
Total current liabilities	96,442	111,266	112,030
Long-term debt, net of current maturities	693,957	696,576	696,576
Deferred rent, net of current portion	7,700	6,127	6,127
Deferred revenue, net of current portion	23,255	21,681	8,440
Deferred tax liabilities	1,389	1,629	1,629
Payable pursuant to tax benefit arrangements, net of current portion	391,876	400,298	400,298
Other liabilities	1,350	4,302	4,302
Total noncurrent liabilities	1,119,527	1,130,613	1,117,372
Stockholders' equity (deficit):
Accumulated other comprehensive loss	(385)	(648)	(648)
Additional paid in capital	14,744	12,118	12,118
Accumulated deficit	(94,348)	(140,158)	(130,966)
Total stockholders' deficit attributable to Planet Fitness Inc.	(79,979)	(128,678)	(119,486)
Non-controlling interests	(11,269)	(17,451)	(17,451)
Total stockholders' deficit	(91,248)	(146,129)	(136,937)
Total liabilities and stockholders' deficit	1,124,721	1,095,750	1,092,465
Class A Common Stock
Stockholders' equity (deficit):
Common stock, value	9	9	9
Class B Common Stock
Stockholders' equity (deficit):
Common stock, value	1	$ 1	1
Calculated under Revenue Guidance in Effect before Topic 606
Current assets:
Cash and cash equivalents	147,784		113,080
Accounts receivable, net	14,932		37,272
Due from related parties	0		3,020
Inventory	3,193		2,692
Restricted assets – national advertising fund	73		499
Deferred expenses – national advertising fund	1,648
Prepaid expenses	3,796		3,929
Other receivables	23,343		9,562
Other current assets	5,916		6,947
Total current assets	200,685		177,001
Property and equipment, net	87,570		83,327
Intangible assets, net	237,092		235,657
Goodwill	191,038		176,981
Deferred income taxes	403,414		407,782
Other assets, net	1,637		11,717
Total assets	1,121,436		1,092,465
Current liabilities:
Current maturities of long-term debt	7,185		7,185
Accounts payable	16,268		28,648
Accrued expenses	15,339		18,590
Equipment deposits	9,001		6,498
Restricted liabilities – national advertising fund	73		490
Deferred revenue, current	23,492		19,083
Payable pursuant to tax benefit arrangements, current	25,578		31,062
Other current liabilities	436		474
Total current liabilities	97,372		112,030
Long-term debt, net of current maturities	693,957		696,576
Deferred rent, net of current portion	7,700		6,127
Deferred revenue, net of current portion	8,187		8,440
Deferred tax liabilities	1,389		1,629
Payable pursuant to tax benefit arrangements, net of current portion	391,876		400,298
Other liabilities	1,350		4,302
Total noncurrent liabilities	1,104,459		1,117,372
Stockholders' equity (deficit):
Accumulated other comprehensive loss	(385)		(648)
Additional paid in capital	14,744		12,118
Accumulated deficit	(83,774)		(130,966)
Total stockholders' deficit attributable to Planet Fitness Inc.	(69,405)		(119,486)
Non-controlling interests	(10,990)		(17,451)
Total stockholders' deficit	(80,395)		(136,937)
Total liabilities and stockholders' deficit	1,121,436		1,092,465
Calculated under Revenue Guidance in Effect before Topic 606 | Class A Common Stock
Stockholders' equity (deficit):
Common stock, value	9		9
Calculated under Revenue Guidance in Effect before Topic 606 | Class B Common Stock
Stockholders' equity (deficit):
Common stock, value	1		1
Total adjustments | Accounting Standards Update 2014-09
Current assets:
Cash and cash equivalents	0		0
Accounts receivable, net	0		0
Due from related parties	0		0
Inventory	0		0
Restricted assets – national advertising fund	0		0
Deferred expenses – national advertising fund	0
Prepaid expenses	0		0
Other receivables	0		0
Other current assets	0		0
Total current assets	0		0
Property and equipment, net	0		0
Intangible assets, net	0		0
Goodwill	0		0
Deferred income taxes	(3,285)		3,285
Other assets, net	0		0
Total assets	(3,285)		3,285
Current liabilities:
Current maturities of long-term debt	0		0
Accounts payable	0		0
Accrued expenses	624		0
Equipment deposits	0		0
Restricted liabilities – national advertising fund	0		0
Deferred revenue, current	306		(764)
Payable pursuant to tax benefit arrangements, current	0		0
Other current liabilities	0		0
Total current liabilities	930		(764)
Long-term debt, net of current maturities	0		0
Deferred rent, net of current portion	0		0
Deferred revenue, net of current portion	(15,068)		13,241
Deferred tax liabilities	0		0
Payable pursuant to tax benefit arrangements, net of current portion	0		0
Other liabilities	0		0
Total noncurrent liabilities	(15,068)		13,241
Stockholders' equity (deficit):
Accumulated other comprehensive loss	0		0
Additional paid in capital	0		0
Accumulated deficit	10,574		(9,192)
Total stockholders' deficit attributable to Planet Fitness Inc.	10,574		(9,192)
Non-controlling interests	279		0
Total stockholders' deficit	10,853		(9,192)
Total liabilities and stockholders' deficit	(3,285)		3,285
Total adjustments | Accounting Standards Update 2014-09 | Class A Common Stock
Stockholders' equity (deficit):
Common stock, value	0		0
Total adjustments | Accounting Standards Update 2014-09 | Class B Common Stock
Stockholders' equity (deficit):
Common stock, value	$ 0		$ 0