Accounting Policies - Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenues	$ 516,753	$ 502,090
Advertising & Marketing Services
Disaggregation of Revenue [Line Items]
Total revenues	264,402	282,939
Subscription
Disaggregation of Revenue [Line Items]
Total revenues	241,575	205,556
Political
Disaggregation of Revenue [Line Items]
Total revenues	2,704	7,606
Other
Disaggregation of Revenue [Line Items]
Total revenues	$ 8,072	$ 5,989