DEBT - 2.50% Convertible Senior Notes Due 2021 (Details) $ / shares in Units, shares in Thousands			1 Months Ended		3 Months Ended			12 Months Ended
	Aug. 13, 2019 USD ($) trading_day $ / shares shares	Sep. 09, 2014 USD ($) trading_day $ / shares	Apr. 30, 2020 USD ($)	Feb. 29, 2020 USD ($)	Dec. 31, 2020 USD ($) $ / shares	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) $ / shares	Dec. 31, 2019 USD ($) shares	Apr. 08, 2020 USD ($)	Feb. 19, 2020 USD ($)
Debt Instrument [Line Items]
Debt instrument, unamortized discount					$ 16,000			$ 16,000	$ 70,699,000
Payment of a prepayment fee								264,731,000	30,000,000
(Loss) Gain on debt extinguishment								(56,113,000)	26,385,000
Amortization of debt discount and debt issuance costs								5,680,000	23,764,000
Liability Component [Abstract]
Total principal amount					84,118,000			84,118,000	427,500,000
Unamortized discount of the liability component					(16,000)			(16,000)	(70,699,000)
Unamortized debt issuance costs					0			0	(5,543,000)
Carrying value					$ 84,102,000			84,102,000	351,258,000
Interest Expense [Abstract]
Amortization of debt discount and debt issuance costs								5,680,000	$ 23,764,000
Interest expense								6,870,000
Common Stock
Debt Instrument [Line Items]
Issuance of common stock in conjunction with the Convertible Note Exchange (in shares) | shares	15,800								15,817
Common Stock | Beneficial Owner
Debt Instrument [Line Items]
Issuance of common stock in conjunction with the Convertible Note Exchange (in shares) | shares	1,700
Convertible debt
Debt Instrument [Line Items]
(Loss) Gain on debt extinguishment								(47,880,000)	$ 26,385,000
Amortization of debt discount and debt issuance costs								1,550,000	15,398,000
Reacquisition of equity component								19,532,000
Interest Expense [Abstract]
Stated coupon interest								637,000	6,708,000
Amortization of debt discount and debt issuance costs								1,550,000	15,398,000
Interest expense								2,187,000	22,106,000
Convertible debt | Additional Paid-In Capital
Debt Instrument [Line Items]
Reacquisition of equity component								$ 19,532,000
Convertible Senior Notes, 2.5% | Convertible debt
Debt Instrument [Line Items]
Interest rate		2.50%			2.50%			2.50%
Par value of debt	$ 145,000,000.0	$ 345,000,000.0			$ 300,000			$ 300,000
Net proceeds from debt offering		334,200,000
Underwriting discount		10,400,000
Offering expenses		$ 400,000
Trading days, number (trading day) | trading_day		20
Consecutive trading days, period (trading day) | trading_day		30
Stock price trigger (in dollars per share) | $ / shares		$ 25.01
Stock price trigger		130.00%			130.00%
Conversion price (in dollars per share) | $ / shares		$ 19.24			$ 19.24			$ 19.24
Consecutive trading-day period		5 days
Maximum product of the closing sale price of shares of the Company's common stock and the applicable conversion rate for such trading day (less than)		98.00%
Conversion rate of common stock		0.0519852
Effective interest rate		9.34%
Liability component of debt		$ 226,000,000.0
Debt instrument, unamortized discount		119,000,000.0			$ 0			$ 0	14,963,000
Equity component of debt	112,800,000	73,300,000
Amount of debt prepaid	200,000,000.0
Extinguishment of debt, cash paid	30,000,000.0
(Loss) Gain on debt extinguishment			$ (3,900,000)	$ (10,300,000)					26,400,000
Equity component									6,200,000
Amortization of debt discount and debt issuance costs	26,100,000
Unamortized balances of debt discount and debt issuance costs	18,900,000
Decrease in carrying amount of equity component	6,200,000
Repurchased aggregate principal amount			$ 42,100,000	$ 102,500,000
Reacquisition of equity component								300,000	4,763,000
Liability Component [Abstract]
Total principal amount					335,000			335,000	145,000,000
Unamortized discount of the liability component		(119,000,000.0)			0			0	(14,963,000)
Unamortized debt issuance costs					0			0	(725,000)
Carrying value					$ 335,000			$ 335,000	129,312,000
Interest Expense [Abstract]
Amortization of debt discount and debt issuance costs	26,100,000
Convertible Senior Notes, 2.5% | Convertible debt | Beneficial Owner
Debt Instrument [Line Items]
Par value of debt	22,000,000.0
Payment of a prepayment fee	$ 3,500,000
Convertible Senior Notes, 2.5% | Convertible debt | Additional Paid-In Capital
Debt Instrument [Line Items]
Equity issuance costs		$ 3,700,000
Reacquisition of equity component									4,763,000
Convertible Senior Notes, 5.0% | Convertible debt
Debt Instrument [Line Items]
Interest rate	5.00%				5.00%			5.00%
Par value of debt	$ 120,000,000.0
Trading days, number (trading day) | trading_day	20
Consecutive trading days, period (trading day) | trading_day	30
Stock price trigger	150.00%
Conversion price (in dollars per share) | $ / shares	$ 3.09
Conversion rate of common stock	0.3235198
Effective interest rate	17.82%
Liability component of debt	$ 65,800,000
Debt instrument, unamortized discount	54,200,000								51,701,000
Equity component of debt	32,600,000
(Loss) Gain on debt extinguishment						$ (12,400,000)	$ (21,300,000)
Equity component									23,999,000
Amortization of debt discount and debt issuance costs								$ 1,273,000	2,583,000
Repurchased aggregate principal amount										$ 34,500,000	$ 85,500,000
Reacquisition of equity component						$ 2,700,000	$ 16,800,000
Liability Component [Abstract]
Total principal amount					$ 0			0	120,000,000
Unamortized discount of the liability component	(54,200,000)								(51,701,000)
Unamortized debt issuance costs									(2,796,000)
Carrying value									65,503,000
Interest Expense [Abstract]
Stated coupon interest								1,090,000	2,250,000
Amortization of debt discount and debt issuance costs								1,273,000	2,583,000
Interest expense								$ 2,363,000	4,833,000
Convertible Senior Notes, 5.0% | Convertible debt | Beneficial Owner
Debt Instrument [Line Items]
Par value of debt	$ 13,200,000
Convertible Senior Notes, 5.0% | Convertible debt | Additional Paid-In Capital
Debt Instrument [Line Items]
Equity component									$ 23,999,000