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 security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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FAIR VALUE - Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Fair value measurements
Cash and cash equivalents	$ 161,705	$ 77,491	$ 50,837	$ 48,921
Marketable securities	1,023,512	347,224
Estimate Of Fair Value Fair Value Disclosure
Fair value measurements
Total	1,185,217	424,715
Fair Value Inputs Level 1
Fair value measurements
Total	92,318	61,297
Fair Value Inputs Level 2
Fair value measurements
Total	1,092,899	363,418
Cash and money market \| Estimate Of Fair Value Fair Value Disclosure
Fair value measurements
Cash and cash equivalents	92,318	61,297
Cash and money market \| Fair Value Inputs Level 1
Fair value measurements
Cash and cash equivalents	92,318	61,297
Corporate bonds \| Estimate Of Fair Value Fair Value Disclosure
Fair value measurements
Marketable securities	415,459	181,305
Corporate bonds \| Fair Value Inputs Level 2
Fair value measurements
Marketable securities	415,459	181,305
U.S. government agency securities \| Estimate Of Fair Value Fair Value Disclosure
Fair value measurements
Cash and cash equivalents	63,694	3,700
Marketable securities	265,131	54,812
U.S. government agency securities \| Fair Value Inputs Level 2
Fair value measurements
Cash and cash equivalents	63,694	3,700
Marketable securities	265,131	54,812
Asset backed securities \| Estimate Of Fair Value Fair Value Disclosure
Fair value measurements
Marketable securities	287,742	94,515
Asset backed securities \| Fair Value Inputs Level 2
Fair value measurements
Marketable securities	287,742	94,515
Commercial Paper \| Estimate Of Fair Value Fair Value Disclosure
Fair value measurements
Cash and cash equivalents	5,693	10,995
Marketable securities	12,076	9,946
Commercial Paper \| Fair Value Inputs Level 2
Fair value measurements
Cash and cash equivalents	5,693	10,995
Marketable securities	12,076	9,946
Certificates of deposit \| Estimate Of Fair Value Fair Value Disclosure
Fair value measurements
Cash and cash equivalents		1,499
Marketable securities	43,104	6,646
Certificates of deposit \| Fair Value Inputs Level 2
Fair value measurements
Cash and cash equivalents		1,499
Marketable securities	$ 43,104	$ 6,646