Fair value of financial instruments - Fair value on a recurring basis (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Fair value of financial instruments
Assets transfers from Level 1 to Level 2	$ 0
Assets transfers into Level 3	0
Assets transfers out of Level 3	0
Liabilities transfers from Level 1 to Level 2	0
Liabilities transfers from Level 2 to Level 1	0
Liabilities transfers into Level 3	0
Liabilities transfers out of Level 3	0
Long term investments	131,330	$ 134,356	[1]
Level 3 | Fair Value
Fair value of financial instruments
Total liabilities	288,000	287,000
Recurring | Fair Value
Fair value of financial instruments
Cash and cash equivalents	928,517	899,509
Debt securities (corporate and government)	269,865	270,136
Long term investments	131,330	134,356
Total assets	1,329,712	1,304,001
Contingent consideration	288,000	287,000
Total liabilities	288,000	287,000
Recurring | Level 1
Fair value of financial instruments
Cash and cash equivalents	928,517	899,509
Long term investments	131,330	134,356
Total assets	1,059,847	1,033,865
Recurring | Level 2
Fair value of financial instruments
Debt securities (corporate and government)	269,865	270,136
Total assets	269,865	270,136
Recurring | Level 3
Fair value of financial instruments
Contingent consideration	288,000	287,000
Total liabilities	$ 288,000	$ 287,000

[1] The condensed consolidated balance sheet at December 31, 2017 has been derived from the audited financial statements at that date.