The Company and Significant Accounting Policies - Additional Information (Detail)		3 Months Ended			9 Months Ended		12 Months Ended
	Nov. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Location shares	Jun. 30, 2018 Station	Sep. 30, 2017 USD ($) shares	Sep. 30, 2018 USD ($) Segment Station Location $ / shares shares	Sep. 30, 2017 USD ($) shares	Dec. 31, 2018	Dec. 31, 2017 USD ($)	Apr. 11, 2018 USD ($)	Jul. 13, 2017 USD ($)
Accounting Policies [Line Items]
Number of reportable segments | Segment					3
Restricted cash		$ 769,000			$ 769,000			$ 222,294,000
Retransmission consent revenue		8,400,000		$ 8,500,000	$ 26,400,000	$ 23,900,000
Number of Class A common stock shares converted | shares					1
Share-based compensation expenses		$ 1,300,000		$ 1,100,000	$ 3,700,000	$ 3,100,000
Shares of dilutive securities not included in computation of diluted income per share | shares		0		277	92,979	11,346
Amount approved under share purchase program									$ 30,000,000	$ 15,000,000
Unamortized debt issuance costs		$ 3,386,000			$ 3,386,000			$ 3,761,000
Interest income related to available-for-sale securities		933,000		$ 256,000	2,885,000	$ 475,000
Federal income tax rate								35.00%
Scenario Plan
Accounting Policies [Line Items]
Federal income tax rate							21.00%
Available-for-Sale Securities
Accounting Policies [Line Items]
Interest income related to available-for-sale securities		900,000			$ 2,700,000
Interest Rate Swap Agreements
Accounting Policies [Line Items]
Interest rate swap agreement beginning date					Dec. 31, 2015
Interest rate swap terminated date					Nov. 28, 2017
2017 Credit Facility
Accounting Policies [Line Items]
Agreement date	Nov. 30, 2017
Additional borrowing capacity	$ 100,000,000
First lien net leverage ratio	4.00%
Certain customary events of default, number of business days to default in the payment of interest on borrowings					3 days
Certain customary events of default, number of days default continue for compliance with other agreement					30 days
Certain customary events of default, number of days default continue for financial statement delivery obligations					10 days
Certain customary events of default, indebtedness aggregate amount					$ 15,000,000
Certain customary events of default, failure in payment of final judgments aggregate amount					$ 15,000,000
Certain customary events of default, failure in payment of final judgments aggregate amount period					30 days
2017 Credit Facility | Eurodollar Rate
Accounting Policies [Line Items]
Variable interest rate basis spread on debt	2.75%
2017 Credit Facility | Base Rate Margin
Accounting Policies [Line Items]
Variable interest rate basis spread on debt	1.75%
2017 Credit Facility | Term Loan B Facility
Accounting Policies [Line Items]
Senior Secured debt	$ 300,000,000
Maturity date of revolving credit facility	Nov. 30, 2024
2013 Credit Facility | Term Loan B Facility
Accounting Policies [Line Items]
Carrying value of term loan		293,600,000			$ 293,600,000
Unamortized debt issuance costs		3,400,000			3,400,000
Estimated fair value of term loan		$ 293,300,000			$ 293,300,000
Class A common stock
Accounting Policies [Line Items]
Amount approved under share purchase program									$ 15,000,000
Number of shares repurchased | shares		0			2,500,000
Average price of repurchased shares | $ / shares					$ 5.08
Aggregate purchase price of repurchased shares					$ 13,000,000
Employee Stock Options
Accounting Policies [Line Items]
Weighted average period for recognition of unrecognized compensation expense					4 months 24 days
Restricted Stock Units
Accounting Policies [Line Items]
Weighted average period for recognition of unrecognized compensation expense					1 year 3 months 18 days
Total unrecognized compensation expense related to grants of restricted stock units		$ 3,700,000			$ 3,700,000
Univision
Accounting Policies [Line Items]
Payment of sales representation fees to television stations		2,200,000		2,400,000	6,400,000	7,100,000
Amount due from television stations for carriage		4,300,000			4,300,000
Retransmission consent revenue		$ 6,500,000		$ 8,300,000	$ 21,500,000	$ 22,900,000
Common stock percentage held by Univision		10.00%			10.00%
UniMas
Accounting Policies [Line Items]
Affiliate advertising minutes per hour for which entity has right to sell					4 minutes 30 seconds
Accounting Standards Update 2016-18
Accounting Policies [Line Items]
Restricted cash		$ 800,000			$ 800,000			$ 222,300,000
Restricted cash used as collateral for letters of credit								800,000
Accounting Standards Update 2016-18 | Federal Communications Commission
Accounting Policies [Line Items]
Proceeds for broadcast spectrum								$ 221,500,000
Minimum | Employee Stock Options
Accounting Policies [Line Items]
Vesting period					1 year
Minimum | Restricted Stock Units
Accounting Policies [Line Items]
Vesting period					1 year
Minimum | Univision
Accounting Policies [Line Items]
Affiliate advertising minutes per hour for which entity has right to sell					4 minutes
Maximum
Accounting Policies [Line Items]
Total unrecognized compensation expense employee stock option plans		$ 100,000			$ 100,000
Maximum | Employee Stock Options
Accounting Policies [Line Items]
Vesting period					4 years
Maximum | Restricted Stock Units
Accounting Policies [Line Items]
Vesting period					4 years
Television
Accounting Policies [Line Items]
Number of stations owned | Station					55
Number of stations relocated | Station			2
Radio
Accounting Policies [Line Items]
Number of stations owned | Station					49
Radio operations stations, number of location | Location		16			16
Advertisements and Syndicate Radio Programming | Minimum
Accounting Policies [Line Items]
Number of stations owned | Station					300