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 debt securities categorized neither as held-to-maturity nor trading.

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FAIR VALUE - Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair value measurements
Cash and cash equivalents	$ 148,695	$ 77,491	$ 39,206	$ 48,921
Marketable securities	893,474	347,224
Fair Value
Fair value measurements
Total	1,042,169	424,715
Level 1
Fair value measurements
Total	101,103	61,297
Level 2
Fair value measurements
Total	941,066	363,418
Long-term debt	4,400	4,500
Cash and money market \| Fair Value
Fair value measurements
Cash and cash equivalents	101,103	61,297
Cash and money market \| Level 1
Fair value measurements
Cash and cash equivalents	101,103	61,297
Corporate bonds \| Fair Value
Fair value measurements
Cash and cash equivalents	1,998
Marketable securities	345,123	181,305
Corporate bonds \| Level 2
Fair value measurements
Cash and cash equivalents	1,998
Marketable securities	345,123	181,305
U.S. government agency securities \| Fair Value
Fair value measurements
Cash and cash equivalents	8,398	3,700
Marketable securities	194,024	54,812
U.S. government agency securities \| Level 2
Fair value measurements
Cash and cash equivalents	8,398	3,700
Marketable securities	194,024	54,812
Asset backed securities \| Fair Value
Fair value measurements
Marketable securities	253,547	94,515
Asset backed securities \| Level 2
Fair value measurements
Marketable securities	253,547	94,515
Commercial paper. \| Fair Value
Fair value measurements
Cash and cash equivalents	11,998	10,995
Marketable securities	51,994	9,946
Commercial paper. \| Level 2
Fair value measurements
Cash and cash equivalents	11,998	10,995
Marketable securities	51,994	9,946
Certificates of deposit \| Fair Value
Fair value measurements
Cash and cash equivalents	25,198	1,499
Marketable securities	48,786	6,646
Certificates of deposit \| Level 2
Fair value measurements
Cash and cash equivalents	25,198	1,499
Marketable securities	$ 48,786	$ 6,646