Fair value of financial instruments - Fair value on a recurring basis (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair value of financial instruments
Long term investments	$ 158,322	$ 31,987	[1]
Level 3 | Fair Value
Fair value of financial instruments
Total liabilities	304,000	301,000
Recurring | Fair Value
Fair value of financial instruments
Cash and cash equivalents	361,823	652,343
Debt securities (corporate and government)	150,132	156,203
Long term investments	158,322	31,987
Total assets	670,277	840,533
Contingent consideration	304,000	301,000
Total liabilities	304,000	301,000
Recurring | Level 1
Fair value of financial instruments
Cash and cash equivalents	361,823	652,343
Long term investments	19,871	31,987
Total assets	381,694	684,330
Recurring | Level 2
Fair value of financial instruments
Debt securities (corporate and government)	150,132	156,203
Long term investments	138,451
Total assets	288,583	156,203
Recurring | Level 3
Fair value of financial instruments
Contingent consideration	304,000	301,000
Total liabilities	$ 304,000	$ 301,000

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