SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Marketable Securities (Details) $ in Thousands	3 Months Ended
Mar. 31, 2018 USD ($) item	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Available-for-sale securities
Number of objectives of the entity's investment strategy \| item	2
Realized gains	$ 30	$ 4
Other than temporary declines in value	$ 0
Minimum contractual term of certain current investments which can be liquidated	1 year
Amortized Cost	$ 895,769		$ 347,913
Gains in Accumulated Other Comprehensive Income (Loss)	4		11
Losses in Accumulated Other Comprehensive Income (Loss)	(2,299)		(700)
Estimated Fair Value	893,474		347,224
Realized losses	0	$ 0
Corporate bonds
Available-for-sale securities
Amortized Cost	346,193		181,639
Gains in Accumulated Other Comprehensive Income (Loss)	3		10
Losses in Accumulated Other Comprehensive Income (Loss)	(1,073)		(344)
Estimated Fair Value	345,123		181,305
Asset backed securities
Available-for-sale securities
Amortized Cost	254,390		94,700
Losses in Accumulated Other Comprehensive Income (Loss)	(843)		(185)
Estimated Fair Value	253,547		94,515
U.S. government agency securities
Available-for-sale securities
Amortized Cost	194,307		54,974
Losses in Accumulated Other Comprehensive Income (Loss)	(283)		(162)
Estimated Fair Value	194,024		54,812
Commercial paper
Available-for-sale securities
Amortized Cost	52,015		9,953
Losses in Accumulated Other Comprehensive Income (Loss)	(21)		(7)
Estimated Fair Value	51,994		9,946
Certificates of deposit
Available-for-sale securities
Amortized Cost	48,864		6,647
Gains in Accumulated Other Comprehensive Income (Loss)	1		1
Losses in Accumulated Other Comprehensive Income (Loss)	(79)		(2)
Estimated Fair Value	$ 48,786		$ 6,646

Amount of realized loss on investment in marketable security, including other-than-temporary impairment (OTTI).

No definition available.

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- Definition

Amount of realized gain on investment in marketable security, including other-than-temporary impairment (OTTI).

+ References

No definition available.

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- Definition

Represents the minimum contractual term of certain current investments which the company has the ability and intent, if necessary, to liquidate in order to support its current operations.

+ References

No definition available.

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- Definition

Represents the number of objectives of the entity's investment strategies.

+ References

No definition available.

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- Definition

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

+ References

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- Definition

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

+ References

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- Definition

This item represents the cost of debt 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 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

Reference 2: 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)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

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

+ 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=75030427&loc=d3e22054-111558

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

Reference 3: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 320
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 320
-Section 50
-Paragraph 2
-Subparagraph (b)-(g)
-URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62557-112803

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- Definition

The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 35
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=82895926&loc=d3e25351-111560

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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