SHORT-TERM INVESTMENTS - Available-for-sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Available-for-sale securities:
Amortized Cost	$ 582,476	$ 616,845
Gross Unrealized Gains	1,752	25
Gross Unrealized Losses	(140)	(2,740)
Estimated Fair Value	584,088	614,130
Corporate notes and bonds
Available-for-sale securities:
Amortized Cost	403,094	390,185
Gross Unrealized Gains	1,480	12
Gross Unrealized Losses	(135)	(2,321)
Estimated Fair Value	404,439	387,876
U.S. agency obligations
Available-for-sale securities:
Amortized Cost	82,969	121,521
Gross Unrealized Gains	79	11
Gross Unrealized Losses	(1)	(203)
Estimated Fair Value	83,047	121,329
U.S. treasury securities
Available-for-sale securities:
Amortized Cost	74,741	60,362
Gross Unrealized Gains	185	2
Gross Unrealized Losses	(1)	(179)
Estimated Fair Value	74,925	60,185
Commercial paper
Available-for-sale securities:
Amortized Cost	16,869	36,849
Gross Unrealized Gains	3
Gross Unrealized Losses	(3)	(28)
Estimated Fair Value	16,869	36,821
Certificates of deposit
Available-for-sale securities:
Amortized Cost	4,803	7,928
Gross Unrealized Gains	5
Gross Unrealized Losses		(9)
Estimated Fair Value	$ 4,808	$ 7,919

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
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Amount of investment in debt and equity securities categorized neither as trading securities nor held-to-maturity securities and intended be sold or mature one year or operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=49167202&loc=d3e26626-111562

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