Summary of Significant Accounting Policies - (Details) - USD ($) $ in Thousands	3 Months Ended									12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Franchise sales and other franchise revenue										$ 24,667	$ 25,131	$ 25,468
Operating income		$ 49,168	$ 12,808	$ 22,797	$ 15,452	$ 14,370	$ 21,142	$ 20,449	$ 15,856	100,225	71,817	$ 73,580
Increase in Total assets		$ 406,562				$ 437,153				406,562	$ 437,153
Forecast | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Franchise sales and other franchise revenue	$ (2,000)
Increase in deferred revenue and deposits	26,000
Increase in Total assets	$ 4,000
Maximum | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Franchise sales and other franchise revenue										(2,000)
Operating income										$ (2,000)