Revenues - Summary of Disaggregation of Broadcast Advertising Revenue by Sales Channel (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation Of Revenue [Line Items]
Total revenue	$ 82,073	$ 74,575	$ 74,329	$ 66,838	$ 73,460	$ 334,555	$ 70,509	$ 57,510	$ 297,815	$ 536,034	$ 258,514
Advertising
Disaggregation Of Revenue [Line Items]
Total revenue									169,009	171,715	194,530
Advertising | Local Direct
Disaggregation Of Revenue [Line Items]
Total revenue									27,908	30,343	30,371
Advertising | Local Agency
Disaggregation Of Revenue [Line Items]
Total revenue									60,840	64,724	72,778
Advertising | National Agency
Disaggregation Of Revenue [Line Items]
Total revenue									$ 80,261	$ 76,648	$ 91,381