STOCK-BASED COMPENSATION - Narrative (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) employees shares	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) employees shares	Sep. 30, 2019 USD ($)
Stock-based compensation
Stock-based compensation expense | $	$ 41,500,000	$ 24,300,000	$ 111,100,000	$ 60,700,000
Incremental share based compensation expense | $	$ 0		$ 0
Number of employees impacted by change in operational milestones | employees	36		36
Non-cash stock-based compensation expense | $	$ 2,400,000
COVID-19
Stock-based compensation
Non-cash stock-based compensation expense | $	1,800,000
Stock Plans
Stock-based compensation
Unrecognized compensation cost | $	285,200,000		$ 285,200,000
Weighted average period for recognition of cost			2 years 9 months 18 days
Merger Agreement with Genomic Health, Inc.
Stock-based compensation
Non-cash stock-based compensation expense | $	300,000		$ 4,200,000
Merger Agreement with Genomic Health, Inc. | Executive Officer
Stock-based compensation
Non-cash stock-based compensation expense | $	$ 1,600,000
Option Plan Shares
Stock-based compensation
Accelerated vesting (in shares) | shares	708
Option Plan Shares | Merger Agreement with Genomic Health, Inc.
Stock-based compensation
Accelerated vesting (in shares) | shares	0		43,480
Option Plan Shares | Merger Agreement with Genomic Health, Inc. | Executive Officer
Stock-based compensation
Accelerated vesting (in shares) | shares	34,619
Restricted Stock Units
Stock-based compensation
Accelerated vesting (in shares) | shares	33,123
Restricted Stock Units | Merger Agreement with Genomic Health, Inc.
Stock-based compensation
Accelerated vesting (in shares) | shares	36,250
Restricted Stock Units | Merger Agreement with Genomic Health, Inc. | Executive Officer
Stock-based compensation
Accelerated vesting (in shares) | shares	6,836
Restricted Stock Units | Merger Agreement with Genomic Health, Inc. | Employee
Stock-based compensation
Accelerated vesting (in shares) | shares	9,786		38,600