MARKETABLE SECURITIES - Schedule of Available For Sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Available-for-sale securities
Amortized Cost	$ 654,124	$ 175,082
Gains in Accumulated Other Comprehensive Income (Loss)	757	12
Losses in Accumulated Other Comprehensive Income (Loss)	(2,474)	(87)
Estimated Fair Value	652,407	175,007
Cash equivalents
Available-for-sale securities
Amortized Cost	123,392	30,320
Gains in Accumulated Other Comprehensive Income (Loss)	36	2
Losses in Accumulated Other Comprehensive Income (Loss)	(58)	0
Estimated Fair Value	123,370	30,322
Short-term Investments
Available-for-sale securities
Amortized Cost	530,732	144,762
Gains in Accumulated Other Comprehensive Income (Loss)	721	10
Losses in Accumulated Other Comprehensive Income (Loss)	(2,416)	(87)
Estimated Fair Value	529,037	144,685
U.S. government agency securities | Cash equivalents
Available-for-sale securities
Amortized Cost	55,299	30,320
Gains in Accumulated Other Comprehensive Income (Loss)	32	2
Losses in Accumulated Other Comprehensive Income (Loss)	0	0
Estimated Fair Value	55,331	30,322
U.S. government agency securities | Short-term Investments
Available-for-sale securities
Amortized Cost	202,628	140,745
Gains in Accumulated Other Comprehensive Income (Loss)	622	10
Losses in Accumulated Other Comprehensive Income (Loss)	0	(73)
Estimated Fair Value	203,250	140,682
Commercial paper. | Cash equivalents
Available-for-sale securities
Amortized Cost	19,955
Gains in Accumulated Other Comprehensive Income (Loss)	3
Losses in Accumulated Other Comprehensive Income (Loss)	(8)
Estimated Fair Value	19,950
Commercial paper. | Short-term Investments
Available-for-sale securities
Amortized Cost	7,964
Gains in Accumulated Other Comprehensive Income (Loss)	0
Losses in Accumulated Other Comprehensive Income (Loss)	(12)
Estimated Fair Value	7,952
Asset backed securities | Cash equivalents
Available-for-sale securities
Amortized Cost	8,695
Gains in Accumulated Other Comprehensive Income (Loss)	0
Losses in Accumulated Other Comprehensive Income (Loss)	(2)
Estimated Fair Value	8,693
Asset backed securities | Short-term Investments
Available-for-sale securities
Amortized Cost	28,269
Gains in Accumulated Other Comprehensive Income (Loss)	28
Losses in Accumulated Other Comprehensive Income (Loss)	(102)
Estimated Fair Value	28,195
Certificates of deposit | Short-term Investments
Available-for-sale securities
Amortized Cost	31,653
Gains in Accumulated Other Comprehensive Income (Loss)	0
Losses in Accumulated Other Comprehensive Income (Loss)	(134)
Estimated Fair Value	31,519
Corporate bonds | Cash equivalents
Available-for-sale securities
Amortized Cost	39,443
Gains in Accumulated Other Comprehensive Income (Loss)	1
Losses in Accumulated Other Comprehensive Income (Loss)	(48)
Estimated Fair Value	39,396
Corporate bonds | Short-term Investments
Available-for-sale securities
Amortized Cost	260,218	4,017
Gains in Accumulated Other Comprehensive Income (Loss)	71	0
Losses in Accumulated Other Comprehensive Income (Loss)	(2,168)	(14)
Estimated Fair Value	$ 258,121	$ 4,003