Accounting policies - Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Total revenues	$ 577,627	$ 536,932	$ 1,261,816	$ 1,053,685
Advertising & Marketing Services
Disaggregation of Revenue [Line Items]
Total revenues	229,083	289,569	524,236	553,971
Subscription
Disaggregation of Revenue [Line Items]
Total revenues	323,475	236,162	656,277	477,737
Political
Disaggregation of Revenue [Line Items]
Total revenues	17,544	3,229	64,931	5,933
Other
Disaggregation of Revenue [Line Items]
Total revenues	$ 7,525	$ 7,972	$ 16,372	$ 16,044