CONVERTIBLE NOTES - Additional information (Details)					1 Months Ended		3 Months Ended			6 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	Jun. 30, 2020 USD ($) $ / shares	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2020 USD ($) $ / shares shares	Jun. 30, 2019 USD ($)
Long-term debt
Net proceeds from issuance										$ 1,125,547,000	$ 729,479,000
Repayments of debt in cash										$ 150,054,000	493,356,000
Issuance of common stock upon convertible notes settlement (in shares) | shares										2,158,991
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							$ 86.94			$ 86.94
Interest expense
Debt issuance costs amortization							$ 1,139,000	$ 645,000		$ 1,961,000	1,330,000
Debt discount amortization							18,946,000	10,074,000		32,677,000	18,468,000
Loss on settlement of convertible notes					$ 8,000,000.0	$ 10,600,000	0	0		7,954,000	10,558,000
Coupon interest expense							2,567,000	1,739,000		4,498,000	3,846,000
Total interest expense on convertible notes							22,652,000	12,458,000		47,090,000	34,202,000
Other interest expense							260,000	254,000		975,000	500,000
Total interest expense							$ 22,912,000	$ 12,712,000		$ 48,065,000	$ 34,702,000
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
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
2025 Convertible notes
Long-term debt
Fixed interest rate (as a percent)							1.00%			1.00%
Repayments of debt in cash					150,100,000
Repayments of debt						493,400,000
Total consideration, allocated to liability component					85,500,000	375,000,000.0
Amount of debt extinguished					100,000,000.0
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			$ 75.43
Interest expense amortization term										4 years 6 months 18 days
2027 Convertible notes
Long-term debt
Fixed interest rate (as a percent)							0.375%			0.375%
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			$ 111.66
Total transaction costs		18,027,000
Transaction costs allocated to liability component		$ 11,395,000
Interest expense amortization term										6 years 8 months 15 days
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%			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			$ 121.84
Total transaction costs	$ 24,453,000
Transaction costs allocated to liability component	$ 16,811,000
Interest expense amortization term										7 years 8 months 1 day