Description of business, basis of presentation and summary of significant accounting policies - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) market station	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Significant Accounting Policies [Line Items]
Number of television stations | station		47
Number of markets In which entity operates | market		39
Write-offs of accounts receivable		$ 1,900,000	$ 3,600,000	$ 1,900,000
Non-cash goodwill impairment charge	$ 332,900,000		15,200,000	8,000,000
Impairment of indefinite-lived intangible assets		0
Other
Significant Accounting Policies [Line Items]
Investments in non-public businesses, non-operating expense		$ 19,400,000	14,800,000
Restricted Stock
Significant Accounting Policies [Line Items]
Requisite service period		4 years
Performance Shares
Significant Accounting Policies [Line Items]
Requisite service period		3 years
Cost of Sales
Significant Accounting Policies [Line Items]
Bad debt expense		$ 2,600,000	5,200,000	2,300,000
Selling, General and Administrative Expenses
Significant Accounting Policies [Line Items]
Advertising costs		5,000,000	$ 7,100,000	$ 9,500,000
Career Builder
Significant Accounting Policies [Line Items]
Non-cash goodwill impairment charge	$ 332,900,000	$ 332,900,000
Minimum | Building and Improvements
Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life (in years)		10 years
Minimum | Machinery, Equipment, and Fixtures
Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life (in years)		3 years
Maximum | Building and Improvements
Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life (in years)		40 years
Maximum | Machinery, Equipment, and Fixtures
Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life (in years)		25 years