Accounting policies - Narrative (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) segment radio_station market station	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Accounting Policies [Abstract]
Number of operating segments | segment	1
Number of reportable segments | segment	1
Number of television stations | station	62
Number of radio stations | radio_station	2
Number of markets In which entity operates | market	51
Programming rights, expense	$ 18,200	$ 13,500
Syndicated programming rights	34,683		$ 49,616
Allowance for doubtful accounts	$ 5,800		$ 3,700