Basis of presentation and summary of significant accounting policies - Narrative (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) brand state	Sep. 29, 2019 USD ($)	Sep. 30, 2020 USD ($) brand segment state	Sep. 29, 2019 USD ($)
Significant Accounting Policies
Number of states in which entity operates (states) | state	46		46
Number of operating segments (segments) | segment			2
Total operating revenues	$ 814,539	$ 376,649	$ 2,530,223	$ 1,168,636
U.K.
Significant Accounting Policies
Number of media brands, more than (brands) | brand	140		140
Advertising and marketing services
Significant Accounting Policies
Total operating revenues	$ 405,227	184,078	$ 1,249,156	582,320
Restatement Adjustment | Advertising and marketing services
Significant Accounting Policies
Total operating revenues		$ 16,600		$ 51,400