Fair value of financial instruments - Fair value on a recurring basis (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Fair value of financial instruments
Transfers from Level 2 to Level 1	$ 50,700
Transfers from Level 1 to Level 2	0
Long term investments	144,425	$ 31,987	[1]
Level 3 | Fair Value
Fair value of financial instruments
Total liabilities	306,000	301,000
Rollforward of Level 3 liabilities
Payments made during the period	(4,356)
Recurring | Fair Value
Fair value of financial instruments
Cash and cash equivalents	451,754	652,343
Debt securities (corporate and government)	156,852	156,203
Long term investments	144,425	31,987
Total assets	753,031	840,533
Contingent consideration	306,000	301,000
Total liabilities	306,000	301,000
Recurring | Level 1
Fair value of financial instruments
Cash and cash equivalents	451,754	652,343
Long term investments	76,268	31,987
Total assets	528,022	684,330
Recurring | Level 2
Fair value of financial instruments
Debt securities (corporate and government)	156,852	156,203
Long term investments	68,157
Total assets	225,009	156,203
Recurring | Level 3
Fair value of financial instruments
Contingent consideration	306,000	301,000
Total liabilities	$ 306,000	$ 301,000

[1] The condensed consolidated balance sheet at December 31, 2016 has been derived from the audited financial statements at that date.