Significant Transactions - Additional Information (Detail) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) Station
Significant Transactions [Line Items]
Cost of revenue - television (spectrum usage rights)		$ 12,340
FCC
Significant Transactions [Line Items]
Revenue from spectrum usage rights	$ 263,900	263,900
Cost of revenue - television (spectrum usage rights)		$ 12,300
Television | FCC
Significant Transactions [Line Items]
Number of stations owned from spectrum usage rights | Station		4