v3.19.3

INDEBTEDNESS - Convertible Senior Notes Narrative (Details) $ / shares in Units, shares in Millions				9 Months Ended		12 Months Ended
	Dec. 31, 2018 USD ($)	May 25, 2018 USD ($) day $ / shares	Mar. 06, 2017 USD ($) day $ / shares	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) shares
Debt Instrument [Line Items]
Principal payment on conversion of senior notes				$ 0	$ 70,047,000
Loss on extinguishment of long-term debt				0	$ 1,625,000
Convertible Debt
Debt Instrument [Line Items]
Conversion price (in USD per share) | $ / shares		$ 77.85	$ 22.95
Carrying amount of equity component	$ 194,392,000			194,392,000		$ 194,392,000
Issuance costs attributable to the liability component	8,040,000			6,025,000		8,040,000
Convertible Debt | 2023 Notes
Debt Instrument [Line Items]
Aggregate principal amount		$ 862,500,000
Interest rate		0.50%
Conversion rate		0.0128456
Conversion price (in USD per share) | $ / shares		$ 77.85
Carrying amount of equity component	154,019,000	$ 155,300,000		$ 154,019,000		154,019,000
Effective interest rate of the liability component		4.69%		4.69%
Discounts and commissions payable		$ 6,000,000.0
Third party offering costs		800,000
Issuance costs attributable to the liability component	5,054,000	$ 5,600,000		$ 3,647,000		5,054,000
Convertible Debt | 2023 Notes | Debt Instrument, Conversion Term One
Debt Instrument [Line Items]
Threshold trading days | day		20
Threshold consecutive trading days | day		30
Threshold percentage of stock price trigger		130.00%
Convertible Debt | 2023 Notes | Debt Instrument Conversion Term Two
Debt Instrument [Line Items]
Threshold trading days | day		5
Threshold consecutive trading days | day		5
Threshold percentage of stock price trigger		98.00%
Convertible Debt | 2022 Notes
Debt Instrument [Line Items]
Aggregate principal amount			$ 440,000,000.0
Interest rate			0.375%
Conversion rate			0.0435749
Conversion price (in USD per share) | $ / shares			$ 22.95
Carrying amount of equity component	40,373,000		$ 86,200,000	$ 40,373,000		40,373,000
Effective interest rate of the liability component			5.34%	5.34%
Discounts and commissions payable			$ 11,000,000.0
Third party offering costs			800,000
Issuance costs attributable to the liability component	2,986,000		$ 9,400,000	$ 2,378,000		2,986,000
Notes converted						228,300,000
Principal payment on conversion of senior notes						$ 219,400,000
Shares issued upon conversion (in shares) | shares						7.3
Loss on extinguishment of long-term debt	5,000,000.0
Reduction to additional paid in capital	$ 21,000,000.0
Convertible Debt | 2022 Notes | Debt Instrument, Conversion Term One
Debt Instrument [Line Items]
Threshold trading days | day			20
Threshold consecutive trading days | day			30
Threshold percentage of stock price trigger			130.00%
Convertible Debt | 2022 Notes | Debt Instrument Conversion Term Two
Debt Instrument [Line Items]
Threshold trading days | day			5
Threshold consecutive trading days | day			5
Threshold percentage of stock price trigger			98.00%

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