Description of business, basis of presentation and summary of significant accounting policies - Revenue Recognition (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Total revenues	$ 2,207,282	$ 1,903,026	$ 2,004,088
Advertising & Marketing Services
Disaggregation of Revenue [Line Items]
Total revenues	1,106,754	1,139,642	1,237,735
Subscription
Disaggregation of Revenue [Line Items]
Total revenues	840,838	718,750	581,733
Political
Disaggregation of Revenue [Line Items]
Total revenues	233,613	23,258	154,808
Other
Disaggregation of Revenue [Line Items]
Total revenues	26,077	21,376	19,844
Former Digital Business
Disaggregation of Revenue [Line Items]
Total revenues	$ 0	$ 0	$ 9,968