Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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FAIR VALUE - Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Fair value measurements
Cash and cash equivalents	$ 50,837	$ 48,921	$ 63,445	$ 41,135
Marketable securities	411,684	262,179
Fair Value
Fair value measurements
Total	462,521	311,100
Level 1
Fair value measurements
Total	30,837	48,921
Level 2
Fair value measurements
Total	431,684	262,179
Long-term debt	4,500	4,600
Cash and Money Market \| Fair Value
Fair value measurements
Cash and cash equivalents	30,837	48,921
Cash and Money Market \| Level 1
Fair value measurements
Cash and cash equivalents	30,837	48,921
Corporate bonds \| Fair Value
Fair value measurements
Marketable securities	213,005	136,937
Corporate bonds \| Level 2
Fair value measurements
Marketable securities	213,005	136,937
U.S. government agency securities \| Fair Value
Fair value measurements
Cash and cash equivalents	20,000
Marketable securities	64,869	49,474
U.S. government agency securities \| Level 2
Fair value measurements
Cash and cash equivalents	20,000
Marketable securities	64,869	49,474
Asset backed securities \| Fair Value
Fair value measurements
Marketable securities	101,113	55,640
Asset backed securities \| Level 2
Fair value measurements
Marketable securities	101,113	55,640
Commercial paper. \| Fair Value
Fair value measurements
Marketable securities	20,895	19,076
Commercial paper. \| Level 2
Fair value measurements
Marketable securities	20,895	19,076
Certificates of deposit \| Fair Value
Fair value measurements
Marketable securities	11,802	1,052
Certificates of deposit \| Level 2
Fair value measurements
Marketable securities	$ 11,802	$ 1,052