Revenue recognition - Cash Flows (Details) - USD ($) $ 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
Cash flows from operating activities:
Net income	$ 28,779	$ 20,472	$ 30,418	$ 23,493	$ 829	$ 18,902	$ 18,004	$ 17,866	$ 103,162	$ 55,601	$ 71,247
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization									35,260	31,761	31,502
Amortization of deferred financing costs									3,400	1,935	1,544
Amortization of favorable leases and asset retirement obligations									375	334	392
Amortization of interest rate caps									1,170	1,755	797
Deferred tax expense									23,933	372,422	15,606
Loss (gain) on re-measurement of tax benefit arrangement									4,765	(317,354)	72
Provision for bad debts									19	(19)	59
Gain on disposal of property and equipment									462	(159)	(514)
Loss on extinguishment of debt									4,570	79	606
Third party debt refinancing expense									0	1,021	3,001
Loss on reacquired franchise rights									360	0	0
Equity-based compensation									5,479	2,531	1,728
Changes in operating assets and liabilities:
Accounts receivable									(1,923)	(10,481)	(7,754)
Due from related parties									3,598
Inventory									(2,430)	(890)	2,755
Other assets and other current assets									5,778	(2,981)	(7,944)
National advertising fund									0
Accounts payable and accrued expenses									14,506	4,210	7,428
Other liabilities and other current liabilities									(2,835)	(470)	2,747
Income taxes									194	(3,027)	(5,993)
Payments pursuant to tax benefit arrangements									(30,493)	(11,446)	(6,922)
Equipment deposits									1,410
Deferred revenue									9,640
Deferred rent									3,999	1,199	632
Net cash provided by operating activities									184,399	$ 131,021	$ 108,817
Total adjustments | Accounting Standards Update 2014-09
Cash flows from operating activities:
Net income									4,654
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization									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
Loss (gain) on re-measurement of tax benefit arrangement									0
Provision for bad debts									0
Gain on disposal of property and equipment									0
Loss on extinguishment of debt									0
Third party debt refinancing expense									0
Loss on reacquired franchise rights									0
Equity-based compensation									0
Changes in operating assets and liabilities:
Accounts receivable									0
Due from related parties									0
Inventory									0
Other assets and other current assets									0
National advertising fund									(425)
Accounts payable and accrued expenses									0
Other liabilities and other current liabilities									0
Income taxes									1,437
Payments pursuant to tax benefit arrangements									0
Equipment deposits									0
Deferred revenue									(5,666)
Deferred rent									0
Net cash provided by operating activities									0
Amounts under Previous Standards
Cash flows from operating activities:
Net income									107,816
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization									35,260
Amortization of deferred financing costs									3,400
Amortization of favorable leases and asset retirement obligations									375
Amortization of interest rate caps									1,170
Deferred tax expense									23,933
Loss (gain) on re-measurement of tax benefit arrangement									4,765
Provision for bad debts									19
Gain on disposal of property and equipment									462
Loss on extinguishment of debt									4,570
Third party debt refinancing expense									0
Loss on reacquired franchise rights									360
Equity-based compensation									5,479
Changes in operating assets and liabilities:
Accounts receivable									(1,923)
Due from related parties									3,598
Inventory									(2,430)
Other assets and other current assets									5,778
National advertising fund									(425)
Accounts payable and accrued expenses									14,506
Other liabilities and other current liabilities									(2,835)
Income taxes									1,631
Payments pursuant to tax benefit arrangements									(30,493)
Equipment deposits									1,410
Deferred revenue									3,974
Deferred rent									3,999
Net cash provided by operating activities									$ 184,399