Other matters - Narrative (Detail)	1 Months Ended	3 Months Ended	12 Months Ended
	Apr. 30, 2017 USD ($) station phase	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) station customer	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Commitments and Contingencies Disclosure [Line Items]
Future minimum payments due under non-cancelable operating leases			$ 109,353,000
Future minimum sublease rentals			4,800,000
Total rental costs reflected in continuing operations			21,000,000	$ 24,600,000	$ 19,200,000
Revenues			$ 1,903,026,000	2,004,088,000	1,764,822,000
FCC Broadcast Spectrum Program, authorized reimbursement amount	$ 1,750,000,000.00
FCC Broadcast Spectrum Program, number of stations affected | station	13		13
FCC Broadcast Spectrum Program, repacking period	39 months
FCC Broadcast Spectrum Program, number of repacking phases | phase	10
FCC Broadcast Spectrum Program, estimated aggregate costs			$ 1,860,000,000
FCC Broadcast Spectrum Program, costs in excess of authorized reimbursement amount			$ 113,900,000
FCC Broadcast Spectrum Program, amounts reimbursed as a percentage of total costs			52.00%
Proceeds from operating agreements		$ 32,600,000	$ 32,588,000	0	$ 0
Deferred revenue, recognition period			20 years
Programming Contracts
Commitments and Contingencies Disclosure [Line Items]
Purchase commitments under contract			$ 1,120,000,000
Capital Projects, Interactive Marketing Agreement, Licensing Fees and Other Commitments
Commitments and Contingencies Disclosure [Line Items]
Purchase commitments under contract			$ 104,600,000
Severance | Voluntary Retirement Program (VRP)
Commitments and Contingencies Disclosure [Line Items]
Restructuring charges				$ 16,000,000
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			$ 215,400,000
Customer B | Sales Revenue | Customer Concentration Risk
Commitments and Contingencies Disclosure [Line Items]
Revenues			$ 202,400,000