Accounting policies - Narrative (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) market station	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) segment radio_station market station	Jun. 30, 2019 USD ($)	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	$ 17,800	$ 13,500	$ 36,000	$ 27,000
Syndicated programming rights	18,786		18,786		$ 49,616
Allowance for doubtful accounts	$ 8,800		$ 8,800		$ 3,700