Accounting policies - Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Total revenues	$ 738,389	$ 551,857	$ 2,000,205	$ 1,605,542
Advertising & Marketing Services
Disaggregation of Revenue [Line Items]
Total revenues	298,605	297,333	822,841	851,304
Subscription
Disaggregation of Revenue [Line Items]
Total revenues	316,677	240,735	972,954	718,472
Political
Disaggregation of Revenue [Line Items]
Total revenues	116,494	8,131	181,425	14,064
Other
Disaggregation of Revenue [Line Items]
Total revenues	$ 6,613	$ 5,658	$ 22,985	$ 21,702