Other matters - Narrative (Detail)	1 Months Ended	3 Months Ended		12 Months Ended
	Apr. 30, 2017 USD ($) station phase	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2018 USD ($) customer	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 23, 2018 USD ($)
Commitments and Contingencies Disclosure [Line Items]
Future minimum payments due under non-cancelable operating leases				$ 127,177,000
Total rental costs reflected in continuing operations				18,500,000	$ 21,000,000	$ 24,600,000
FCC Broadcast Spectrum Program, number of stations affected | station	13
FCC Broadcast Spectrum Program, authorized reimbursement amount	$ 1,750,000,000.00
FCC Broadcast Spectrum Program, repacking fund							$ 1,000,000,000
FCC Broadcast Spectrum Program, repacking fund available for distributors							$ 750,000,000
FCC Broadcast Spectrum Program, repacking period	39 months
FCC Broadcast Spectrum Program, number of repacking phases | phase	10
FCC Broadcast Spectrum Program, capital expenditures incurred				17,600,000
FCC Broadcast Spectrum Program, amount purchased				16,300,000
FCC Broadcast Spectrum Program, amount of reimbursement received				7,400,000
FCC Broadcast Spectrum Program, amounts reimbursed as a percentage of total costs	92.50%
Proceeds from operating agreements			$ 32,600,000	$ 0	32,588,000	$ 0
Deferred revenue, recognition period				20 years
Severance charge		$ 7,300,000		$ 4,900,000
Programming Contracts
Commitments and Contingencies Disclosure [Line Items]
Purchase commitments under contract				1,260,000,000
Capital Projects, Interactive Marketing Agreement, Licensing Fees and Other Commitments
Commitments and Contingencies Disclosure [Line Items]
Purchase commitments under contract				$ 113,300,000
Sales Revenue | Customer Concentration Risk
Commitments and Contingencies Disclosure [Line Items]
Number of customers which represent significant percentage of revenue | customer				2
Customer A | Sales Revenue | Customer Concentration Risk
Commitments and Contingencies Disclosure [Line Items]
Revenues				$ 245,300,000	215,400,000
Customer B | Sales Revenue | Customer Concentration Risk
Commitments and Contingencies Disclosure [Line Items]
Revenues				$ 223,800,000	$ 202,400,000