Other matters (Details)					1 Months Ended			3 Months Ended		9 Months Ended		12 Months Ended	21 Months Ended	29 Months Ended
	Jul. 03, 2020 station	Mar. 02, 2020 USD ($) market	Jun. 17, 2019 defendant	Jul. 30, 2018 defendant	Jun. 30, 2019 defendant	Dec. 13, 2018 defendant	Apr. 30, 2017 USD ($) market	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) station	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) station	Sep. 30, 2020 USD ($)	Sep. 30, 2020 USD ($)	Mar. 23, 2018 USD ($)
Business Acquisition [Line Items]
Number of markets where stations were repacked | market							18
Broadcast spectrum program, number of station acquired | station										1		4
Number of stations broadcasting on interim antennas | station	1
Authorized reimbursement amount							$ 1,750,000,000
Authorized reimbursement amount, repacking fund															$ 1,000,000,000
Authorized reimbursement amount, repacking fund, full power, class a stations and multichannel distributors															$ 750,000,000
Capital expenditures incurred													$ 41,700,000
Broadcast spectrum program, amount paid										$ 6,100,000
FCC reimbursements received														$ 37,000,000.0
Remaining FFC reimbursements expected to be received										4,700,000
Payments for acquisition of minority interest		$ 14,000,000.0								14,000,000	$ 0
Settled litigation
Business Acquisition [Line Items]
Number of other broadcasters settling DOJ complaint (defendant) | defendant			4			7
Equity And Debt Investment | Affiliated Entity
Business Acquisition [Line Items]
Expenses incurred with related party								$ 8,500,000	$ 5,800,000	32,700,000	$ 21,600,000
Accounts payable and accrued liabilities with related party								$ 10,900,000		$ 10,900,000		$ 4,300,000	$ 10,900,000	$ 10,900,000
Clay, Massey & Associates, P.C. v. Gray Television
Business Acquisition [Line Items]
Number of other broadcasters settling DOJ complaint (defendant) | defendant				16	4
Gray Television
Business Acquisition [Line Items]
Number of television markets | market		93