Goodwill and Intangible Assets - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) store	Mar. 31, 2018 USD ($) store	Dec. 31, 2018 USD ($) store	Aug. 10, 2018 store	Jan. 01, 2018 store	Dec. 31, 2017 store
Goodwill And Intangible Assets [Line Items]
Impairment charges | $	$ 0		$ 0
Number of owned and franchised locations | store	1,806	1,565	1,742			1,518
Amortization of intangible assets | $	$ 4,005,000	$ 3,966,000
Favorable and Unfavorable Leases
Goodwill And Intangible Assets [Line Items]
Amortization of intangible assets | $		$ 93,000
Long Island Acquisition
Goodwill And Intangible Assets [Line Items]
Number of owned and franchised locations | store					6
Colorado Acquisition
Goodwill And Intangible Assets [Line Items]
Number of owned and franchised locations | store				4