Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary Of Significant Accounting Policies [Line Items]
Marketing and advertising costs	$ 197.6	$ 161.1	$ 169.1
Contractual Relationship [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life	20 years
Customer and advertiser lists [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life	25 years
Tradenames [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life	25 years
Broadcast licenses [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life	25 years
Acquired Rights [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life	25 years
Other Intangible Assets [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life	25 years
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Ownership interest (in hundredths)	20.00%
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Ownership interest (in hundredths)	50.00%