Fair value of financial instruments - Fair value on a recurring basis (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fair value of financial instruments
Long term investment	$ 55,745	$ 35,248	[1]
Level 3 | Fair Value
Fair value of financial instruments
Total liabilities	298,000
Recurring | Fair Value
Fair value of financial instruments
Cash and cash equivalents	580,099	521,439
Corporate debt securities	136,486	186,344
Long term investment	55,745	35,248
Total assets	772,330	743,031
Contingent consideration	298,000
Total liabilities	298,000
Recurring | Level 1
Fair value of financial instruments
Cash and cash equivalents	580,099	521,439
Long term investment	55,745	35,248
Total assets	635,844	556,687
Recurring | Level 2
Fair value of financial instruments
Corporate debt securities	136,486	186,344
Total assets	136,486	$ 186,344
Recurring | Level 3
Fair value of financial instruments
Contingent consideration	298,000
Total liabilities	$ 298,000

[1] The condensed consolidated balance sheet at December 31, 2015 has been derived from the audited financial statements at that date.