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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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Fair value portion of contractual obligation to pay money on demand or on fixed or determinable dates.

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FAIR VALUE - Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Fair value measurements
Cash and cash equivalents	$ 52,151	$ 41,135	$ 40,679	$ 58,131
Marketable securities	210,441	265,744
Estimate Of Fair Value Fair Value Disclosure
Fair value measurements
Total	262,592	306,879
Fair Value Inputs Level 1
Fair value measurements
Total	38,151	37,435
Fair Value Inputs Level 2
Fair value measurements
Total	224,441	269,444
Long-term debt	4,800	4,800
Cash and Money Market \| Estimate Of Fair Value Fair Value Disclosure
Fair value measurements
Cash and cash equivalents	38,151	37,435
Cash and Money Market \| Fair Value Inputs Level 1
Fair value measurements
Cash and cash equivalents	38,151	37,435
Corporate bonds \| Estimate Of Fair Value Fair Value Disclosure
Fair value measurements
Marketable securities	132,906	179,211
Corporate bonds \| Fair Value Inputs Level 2
Fair value measurements
Marketable securities	132,906	179,211
U.S. government agency securities \| Estimate Of Fair Value Fair Value Disclosure
Fair value measurements
Marketable securities	7,065	7,039
U.S. government agency securities \| Fair Value Inputs Level 2
Fair value measurements
Marketable securities	7,065	7,039
Asset backed securities \| Estimate Of Fair Value Fair Value Disclosure
Fair value measurements
Marketable securities	62,348	77,495
Asset backed securities \| Fair Value Inputs Level 2
Fair value measurements
Marketable securities	62,348	77,495
Commercial Paper. \| Estimate Of Fair Value Fair Value Disclosure
Fair value measurements
Cash and cash equivalents		3,700
Marketable securities	2,620
Commercial Paper. \| Fair Value Inputs Level 2
Fair value measurements
Cash and cash equivalents		3,700
Marketable securities	2,620
Certificates of deposit \| Estimate Of Fair Value Fair Value Disclosure
Fair value measurements
Cash and cash equivalents	14,000
Marketable securities	5,502	1,999
Certificates of deposit \| Fair Value Inputs Level 2
Fair value measurements
Cash and cash equivalents	14,000
Marketable securities	$ 5,502	$ 1,999