Description of business, basis of presentation and summary of significant accounting policies - Additional Information (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) market station	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2018 USD ($)
Significant Accounting Policies [Line Items]
Number of television stations | station		49
Number of radio stations | station		2
Number of markets In which entity operates | market		41
Write-offs of accounts receivable		$ 3,900,000	$ 1,900,000	$ 3,600,000
Construction in progress		40,778,000	5,535,000
Impairment of indefinite-lived intangible assets		0	0	6,300,000
Investments in non-public businesses, non-operating expense		24,500,000	17,400,000
Debt Investment, Impairment Charges		2,000,000
Impairment charge to cost method investments		2,000,000	2,600,000
Cost of revenues, exclusive of depreciation		1,065,933,000	933,718,000	795,454,000
Cumulative effect of accounting change			(3,724,000)
Life insurance proceeds		$ 0
Restricted Cash	$ 0		29,200,000	28,200,000
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		$ 3,900,000	2,600,000	5,200,000
Minimum
Significant Accounting Policies [Line Items]
Submission period for customers to submit payments	P60D
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
Significant Accounting Policies [Line Items]
Submission period for customers to submit payments	P90D
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
Advertising | Selling, General and Administrative Expenses
Significant Accounting Policies [Line Items]
Cost of revenues, exclusive of depreciation		$ 10,400,000	5,000,000	7,100,000
Accumulated other comprehensive income (loss)
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change			(24,845,000)
Retained earnings
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change			21,121,000
Accounting Standards Update 2014-09 | Barter Arrangement, Syndicated Programming
Significant Accounting Policies [Line Items]
Revenues		$ 2,000,000
Accounting Standards Update 2014-09 | Retained earnings
Significant Accounting Policies [Line Items]
Cumulative effect of accounting change					$ (3,700,000)
Previously Reported | Accounting Standards Update 2016-15
Significant Accounting Policies [Line Items]
Reclassification of life insurance proceeds from operating inflows			1,400,000	500,000
Reclassification of debt prepayment costs from operating cash outflows			3,600,000
Reclassification of life insurance proceeds to investing inflows			1,400,000	$ 500,000
Reclassification of debt prepayment costs to financing cash outflows			$ 3,600,000