Accounting policies - Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Total revenues	$ 684,189	$ 516,753
Advertising & Marketing Services
Disaggregation of Revenue [Line Items]
Total revenues	295,153	264,402
Subscription
Disaggregation of Revenue [Line Items]
Total revenues	332,802	241,575
Political
Disaggregation of Revenue [Line Items]
Total revenues	47,387	2,704
Other
Disaggregation of Revenue [Line Items]
Total revenues	$ 8,847	$ 8,072