SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Marketable Securities (Details)	9 Months Ended
	Sep. 30, 2018 USD ($) item	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Available-for-sale securities
Number of objectives of the entity's investment strategy | item	2
Realized gains	$ 200,000	$ 17,000
Other than temporary declines in value	$ 0
Minimum contractual term of certain current investments which can be liquidated	1 year
Amortized Cost	$ 1,024,869,000		$ 347,913,000
Gains in Accumulated Other Comprehensive Income (Loss)	122,000		11,000
Losses in Accumulated Other Comprehensive Income (Loss)	(1,479,000)		(700,000)
Estimated Fair Value	1,023,512,000		347,224,000
Realized losses	0	$ 0
Corporate bonds
Available-for-sale securities
Amortized Cost	415,828,000		181,639,000
Gains in Accumulated Other Comprehensive Income (Loss)	104,000		10,000
Losses in Accumulated Other Comprehensive Income (Loss)	(473,000)		(344,000)
Estimated Fair Value	415,459,000		181,305,000
Asset backed securities
Available-for-sale securities
Amortized Cost	288,286,000		94,700,000
Gains in Accumulated Other Comprehensive Income (Loss)	3,000
Losses in Accumulated Other Comprehensive Income (Loss)	(547,000)		(185,000)
Estimated Fair Value	287,742,000		94,515,000
U.S. government agency securities
Available-for-sale securities
Amortized Cost	265,563,000		54,974,000
Losses in Accumulated Other Comprehensive Income (Loss)	(432,000)		(162,000)
Estimated Fair Value	265,131,000		54,812,000
Commercial Paper Not Included With Cash And Cash Equivalents
Available-for-sale securities
Amortized Cost	12,081,000		9,953,000
Losses in Accumulated Other Comprehensive Income (Loss)	(5,000)		(7,000)
Estimated Fair Value	12,076,000		9,946,000
Certificates of deposit
Available-for-sale securities
Amortized Cost	43,111,000		6,647,000
Gains in Accumulated Other Comprehensive Income (Loss)	15,000		1,000
Losses in Accumulated Other Comprehensive Income (Loss)	(22,000)		(2,000)
Estimated Fair Value	$ 43,104,000		$ 6,646,000