CONVERTIBLE NOTES - Additional information (Details)					1 Months Ended		3 Months Ended
	Feb. 29, 2020 USD ($)	Mar. 08, 2019 USD ($)	Jun. 12, 2018 USD ($)	Jan. 17, 2018 USD ($)	Feb. 29, 2020 USD ($)	Mar. 31, 2019 USD ($) shares	Mar. 31, 2020 USD ($) $ / shares shares	Mar. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)
Long-term debt
Net proceeds from issuance							$ 1,125,547,000	$ 729,536,000
Repayments of debt in cash							$ 150,054,000	493,355,000
Issuance of common stock upon convertible notes settlement (in shares) | shares							2,159,716
Shares issued to settle convertible notes								182,435,000
Retirement of equity component of convertible notes settled							$ (64,199,000)	(300,768,000)
Closing price of common stock | $ / shares							$ 58.00
Interest expense
Debt issuance costs amortization							$ 822,000	685,000
Debt discount amortization							13,731,000	8,394,000
Loss on settlement of convertible notes					$ 8,000,000.0	$ 10,600,000	7,954,000	10,558,000
Coupon interest expense							1,932,000	2,108,000
Interest Expense, Debt, Total							24,439,000	21,745,000
Other interest expense							714,000	245,000
Total interest expense							$ 25,153,000	21,990,000
Convertible Notes
Long-term debt
Fixed interest rate (as a percent)				1.00%
Net proceeds from issuance				$ 671,100,000
Notes
Long-term debt
Amount issued and sold		$ 747,500,000
Fixed interest rate (as a percent)		0.375%
Net proceeds from issuance		$ 729,500,000
Repurchase price, as percentage of principal amount, if company undergoes change of control							100
2025 Notes
Long-term debt
Repayments of Debt						493,400,000
Total consideration, allocated to liability component	$ 85,500,000				85,500,000				$ 375,000,000.0
Retirement of equity component of convertible notes settled	64,200,000							$ 300,800,000
Debt instrument, convertible, equity component, tax impact					300,000
Amount used to pay off interest accrued					100,000	700,000
Conversion rate, number of shares to be issued per $1,000 of principal amount (in shares)							13.26
Conversion price (in dollars per share) | $ / shares							$ 75.43
2027 Notes
Long-term debt
Repayments of debt in cash						$ 494,100,000
Issuance of common stock upon convertible notes settlement (in shares) | shares						2,200,000
Shares issued to settle convertible notes						$ 182,400,000
Total consideration						$ 676,500,000
Conversion rate, number of shares to be issued per $1,000 of principal amount (in shares)							8.96
Conversion price (in dollars per share) | $ / shares							$ 111.66
Total transaction costs		18,027,000
Transaction costs allocated to liability component		$ 11,395,000
2028 Convertible Notes
Long-term debt
Amount issued and sold	$ 1,150,000,000.0				$ 1,150,000,000.0
Fixed interest rate (as a percent)	0.375%				0.375%		0.375%
Net proceeds from issuance					$ 1,125,600,000
Conversion rate, number of shares to be issued per $1,000 of principal amount (in shares)							8.21
Conversion price (in dollars per share) | $ / shares							$ 121.84
Total transaction costs	$ 24,453,000
Transaction costs allocated to liability component	$ 16,811,000
Interest expense amortization term							7 years 11 months 1 day
January 2018 Notes
Long-term debt
Amount issued and sold				$ 690,000,000.0
Fixed interest rate (as a percent)				1.00%
June 2018 Notes
Long-term debt
Amount issued and sold			$ 218,500,000
Fixed interest rate (as a percent)			1.00%
Net proceeds from issuance			$ 225,300,000
2027 Convertible notes
Long-term debt
Fixed interest rate (as a percent)							0.375%
Interest expense amortization term							6 years 11 months 15 days
2025 Convertible notes
Long-term debt
Fixed interest rate (as a percent)							1.00%
Repayments of debt in cash					150,100,000
Amount of debt extinguished					$ 100,000,000.0
Interest expense amortization term							4 years 9 months 18 days