Accounting Standards Adopted and Newly Issued Accounting Standards Not Yet Adopted - Reclassified franchise and restaurant revenues and expenses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Franchise revenues	$ 151,941	$ 153,064	$ 307,254	$ 307,789
Company restaurants	0	3,378	0	7,518
Franchise expenses	82,944	69,522	164,816	139,689
Company restaurant expenses	0	3,447	0	7,790
As Reported
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Franchise and Restaurant Revenues		122,987		246,565
Franchise revenues		119,609		239,047
Company restaurants		3,378		7,518
Franchise and Restaurant Expenses		40,669		81,676
Franchise expenses		37,222		73,886
Company restaurant expenses		3,447		7,790
Operating Segments | Company restaurants
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Company restaurants	$ 0	$ 3,400	$ 0	$ 7,500