Revenue recognition - Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities:
Net income	$ 30,418	$ 18,004	$ 53,912	$ 35,870
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	8,619	$ 7,894	17,084	15,845
Amortization of deferred financing costs			973	942
Amortization of favorable leases and asset retirement obligations			186	184
Amortization of interest rate caps			446	954
Deferred tax expense			13,300	14,589
Gain on re-measurement of tax benefit arrangement			(354)	(541)
Provision for bad debts			(8)	28
Loss on reacquired franchise rights			350	0
Loss (gain) on disposal of property and equipment			547	(323)
Equity-based compensation			2,687	1,012
Changes in operating assets and liabilities, excluding effects of acquisitions:
Accounts receivable			22,281	11,542
Due to and due from related parties			3,375	(289)
Inventory			(501)	355
Other assets and other current assets			(3,109)	(3,239)
National advertising fund			(1,634)	0
Accounts payable and accrued expenses			(16,884)	(14,144)
Other liabilities and other current liabilities			(2,908)	(33)
Income taxes			131	(406)
Payable to related parties pursuant to tax benefit arrangements			(21,706)	(7,909)
Equipment deposits			2,503	5,390
Deferred revenue			6,229	1,826
Deferred rent			1,594	245
Net cash provided by operating activities			78,494	$ 62,998
Total adjustments | Accounting Standards Update 2014-09
Cash flows from operating activities:
Net income	562		1,903
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	0		0
Amortization of deferred financing costs			0
Amortization of favorable leases and asset retirement obligations			0
Amortization of interest rate caps			0
Deferred tax expense			0
Gain on re-measurement of tax benefit arrangement			0
Provision for bad debts			0
Loss on reacquired franchise rights			0
Loss (gain) on disposal of property and equipment			0
Equity-based compensation			0
Changes in operating assets and liabilities, excluding effects of acquisitions:
Accounts receivable			0
Due to and due from related parties			0
Inventory			0
Other assets and other current assets			0
National advertising fund			0
Accounts payable and accrued expenses			0
Other liabilities and other current liabilities			0
Income taxes			624
Payable to related parties pursuant to tax benefit arrangements			0
Equipment deposits			0
Deferred revenue			(2,527)
Deferred rent			0
Net cash provided by operating activities			0
Calculated under Revenue Guidance in Effect before Topic 606
Cash flows from operating activities:
Net income	30,980		55,815
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	$ 8,619		17,084
Amortization of deferred financing costs			973
Amortization of favorable leases and asset retirement obligations			186
Amortization of interest rate caps			446
Deferred tax expense			13,300
Gain on re-measurement of tax benefit arrangement			(354)
Provision for bad debts			(8)
Loss on reacquired franchise rights			350
Loss (gain) on disposal of property and equipment			547
Equity-based compensation			2,687
Changes in operating assets and liabilities, excluding effects of acquisitions:
Accounts receivable			22,281
Due to and due from related parties			3,375
Inventory			(501)
Other assets and other current assets			(3,109)
National advertising fund			(1,634)
Accounts payable and accrued expenses			(16,884)
Other liabilities and other current liabilities			(2,908)
Income taxes			755
Payable to related parties pursuant to tax benefit arrangements			(21,706)
Equipment deposits			2,503
Deferred revenue			3,702
Deferred rent			1,594
Net cash provided by operating activities			$ 78,494