Accounting policies - Narrative (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) station market	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) segment station radio_station market	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Accounting Policies [Abstract]
Number of operating segments | segment			1
Number of reportable segments | segment			1
Number of television stations | station	63		63
Number of radio stations | radio_station			2
Number of markets In which entity operates | market	51		51
Programming rights, expense	$ 17,600	$ 15,500	$ 53,600	$ 42,500
Syndicated programming rights	60,378		60,378		$ 49,616
Allowance for doubtful accounts	$ 8,400		$ 8,400		$ 3,700