Corporate-owned and franchisee-owned stores - Schedule of Changes in Corporate-owned and Franchisee-owned Stores (Detail) - store	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Franchisor Disclosure [Line Items]
Stores operated at beginning of period	1,518	1,313	1,124
New stores opened	230	210	195
Stores debranded, sold or consolidated	(6)	(5)	(6)
Stores operated at end of period	1,742	1,518	1,313
Franchisee-owned stores
Franchisor Disclosure [Line Items]
Stores operated at beginning of period	1,456	1,255	1,066
New stores opened	226	206	195
Stores debranded, sold or consolidated	(16)	(5)	(6)
Stores operated at end of period	1,666	1,456	1,255
Corporate-owned stores
Franchisor Disclosure [Line Items]
Stores operated at beginning of period	62	58	58
New stores opened	4	4	0
Stores acquired from franchisees	10	0	0
Stores operated at end of period	76	62	58