STOCK AWARD PLANS - Summary of Total Compensation Expense Recognized in Statements of Operations for Stock-Based Compensation (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 8,850,000	$ 23,771,000	$ 40,657,000	$ 68,475,000
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	4,829,000	5,666,000	13,539,000	17,931,000
Stock awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	3,470,000	17,539,000	24,798,000	48,443,000
Company expense on ESPP discount
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 550,787	$ 566,000	$ 2,320,000	$ 2,101,000