FAIR VALUE MEASUREMENTS - Schedule of Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair value measurements
Cash and cash equivalents	$ 701,334	$ 177,528
Estimated Fair Value	652,407	175,007
Equity securities	1,025	1,716
Restricted cash
Fair value measurements
Cash and cash equivalents	280	274
Fair Value, Recurring
Fair value measurements
Equity securities	1,025	1,716
Contingent consideration	(2,739)	(2,879)
Total	1,228,657	321,050
Fair Value, Recurring | Corporate bonds
Fair value measurements
Estimated Fair Value	258,121	4,003
Fair Value, Recurring | Asset backed securities
Fair value measurements
Estimated Fair Value	28,195
Fair Value, Recurring | U.S. government agency securities
Fair value measurements
Estimated Fair Value	203,250	140,682
Fair Value, Recurring | Certificates of deposit
Fair value measurements
Estimated Fair Value	31,519
Fair Value, Recurring | Commercial paper
Fair value measurements
Estimated Fair Value	7,952
Fair Value, Recurring | Cash and money market
Fair value measurements
Cash and cash equivalents	577,684	146,932
Fair Value, Recurring | U.S. government agency securities
Fair value measurements
Cash and cash equivalents	55,331	30,322
Fair Value, Recurring | Commercial paper
Fair value measurements
Cash and cash equivalents	19,950
Fair Value, Recurring | Certificates of deposit
Fair value measurements
Cash and cash equivalents	39,396
Fair Value, Recurring | Asset backed securities
Fair value measurements
Cash and cash equivalents	8,693
Fair Value, Recurring | Restricted cash
Fair value measurements
Cash and cash equivalents	280	274
Level 1 | Fair Value, Recurring
Fair value measurements
Equity securities	1,025	1,716
Contingent consideration	0	0
Total	578,989	148,922
Level 1 | Fair Value, Recurring | Corporate bonds
Fair value measurements
Estimated Fair Value	0	0
Level 1 | Fair Value, Recurring | Asset backed securities
Fair value measurements
Estimated Fair Value	0
Level 1 | Fair Value, Recurring | U.S. government agency securities
Fair value measurements
Estimated Fair Value	0	0
Level 1 | Fair Value, Recurring | Certificates of deposit
Fair value measurements
Estimated Fair Value	0
Level 1 | Fair Value, Recurring | Commercial paper
Fair value measurements
Estimated Fair Value	0
Level 1 | Fair Value, Recurring | Cash and money market
Fair value measurements
Cash and cash equivalents	577,684	146,932
Level 1 | Fair Value, Recurring | U.S. government agency securities
Fair value measurements
Cash and cash equivalents	0	0
Level 1 | Fair Value, Recurring | Commercial paper
Fair value measurements
Cash and cash equivalents	0
Level 1 | Fair Value, Recurring | Certificates of deposit
Fair value measurements
Cash and cash equivalents	0
Level 1 | Fair Value, Recurring | Asset backed securities
Fair value measurements
Cash and cash equivalents	0
Level 1 | Fair Value, Recurring | Restricted cash
Fair value measurements
Cash and cash equivalents	280	274
Level 2 | Fair Value, Recurring
Fair value measurements
Equity securities	0	0
Contingent consideration	0	0
Total	652,407	175,007
Level 2 | Fair Value, Recurring | Corporate bonds
Fair value measurements
Estimated Fair Value	258,121	4,003
Level 2 | Fair Value, Recurring | Asset backed securities
Fair value measurements
Estimated Fair Value	28,195
Level 2 | Fair Value, Recurring | U.S. government agency securities
Fair value measurements
Estimated Fair Value	203,250	140,682
Level 2 | Fair Value, Recurring | Certificates of deposit
Fair value measurements
Estimated Fair Value	31,519
Level 2 | Fair Value, Recurring | Commercial paper
Fair value measurements
Estimated Fair Value	7,952
Level 2 | Fair Value, Recurring | Cash and money market
Fair value measurements
Cash and cash equivalents	0	0
Level 2 | Fair Value, Recurring | U.S. government agency securities
Fair value measurements
Cash and cash equivalents	55,331	30,322
Level 2 | Fair Value, Recurring | Commercial paper
Fair value measurements
Cash and cash equivalents	19,950
Level 2 | Fair Value, Recurring | Certificates of deposit
Fair value measurements
Cash and cash equivalents	39,396
Level 2 | Fair Value, Recurring | Asset backed securities
Fair value measurements
Cash and cash equivalents	8,693
Level 2 | Fair Value, Recurring | Restricted cash
Fair value measurements
Cash and cash equivalents	0	0
Level 3 | Fair Value, Recurring
Fair value measurements
Equity securities	0	0
Contingent consideration	(2,739)	(2,879)
Total	(2,739)	(2,879)
Level 3 | Fair Value, Recurring | Corporate bonds
Fair value measurements
Estimated Fair Value	0	0
Level 3 | Fair Value, Recurring | Asset backed securities
Fair value measurements
Estimated Fair Value	0
Level 3 | Fair Value, Recurring | U.S. government agency securities
Fair value measurements
Estimated Fair Value	0	0
Level 3 | Fair Value, Recurring | Certificates of deposit
Fair value measurements
Estimated Fair Value	0
Level 3 | Fair Value, Recurring | Commercial paper
Fair value measurements
Estimated Fair Value	0
Level 3 | Fair Value, Recurring | Cash and money market
Fair value measurements
Cash and cash equivalents	0	0
Level 3 | Fair Value, Recurring | U.S. government agency securities
Fair value measurements
Cash and cash equivalents	0	0
Level 3 | Fair Value, Recurring | Commercial paper
Fair value measurements
Cash and cash equivalents	0
Level 3 | Fair Value, Recurring | Certificates of deposit
Fair value measurements
Cash and cash equivalents	0
Level 3 | Fair Value, Recurring | Asset backed securities
Fair value measurements
Cash and cash equivalents	0
Level 3 | Fair Value, Recurring | Restricted cash
Fair value measurements
Cash and cash equivalents	$ 0	$ 0