Convertible Senior Notes - Additional Information (Detail)			1 Months Ended	3 Months Ended		9 Months Ended
	Jul. 22, 2019 USD ($) shares	Jul. 19, 2019 USD ($) $ / shares	Sep. 23, 2019 USD ($) shares	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jul. 31, 2019 USD ($)	Jul. 16, 2019 USD ($)
Debt Instrument [Line Items]
Proceeds from issuance of convertible senior notes, net of costs							$ 278,555,000
Accretion of the debt discount						$ 8,175,000	4,863,000
Total consideration on conversion							198,757,000
Loss on extinguishment of debt					$ (5,650,000)	5,700,000	(5,650,000)
Common Stock [Member]
Debt Instrument [Line Items]
Conversion of Convertible Securities Stock Issued | shares | shares				2,316,200
0.375% Convertible Senior Notes due 2024
Debt Instrument [Line Items]
Notes issued		$ 287,500,000						$ 287,500,000
Notes, interest rate		0.375%
Proceeds from issuance of convertible senior notes, net of costs		$ 278,500,000
Interest repayment terms		Interest is payable semi-annually in arrears on January 15 and July 15 of each year, beginning on January 15, 2020.
Notes, due date		Jul. 15, 2024
Notes conversion ratio per $1,000 principal amount		8.6749
Notes initial conversion price | $ / shares		$ 115.28
Notes redemption price		100.00%
Contractual coupon interest				$ 300,000		800,000
Accretion of the debt discount				2,400,000		7,200,000
Amortization of the debt issuance costs				$ 300,000		$ 1,000,000.0
Effective interest rate on the Notes				5.10%		5.10%
Notes, carrying value				$ 240,900,000		$ 240,900,000
Fair value of the note				416,000,000.0		$ 416,000,000.0
Additional Notes issued		$ 37,500,000
Discount rate						4.50%
Proceeds allocated to the conversion feature						$ 52,100,000
Transaction costs attributable to liability component						7,400,000
Transaction costs attributable to equity component				1,600,000		1,600,000
2.125% Convertible Senior Notes due 2021
Debt Instrument [Line Items]
Notes issued				115,000,000.0		115,000,000.0			$ 92,000,000.0
Notes, interest rate									2.125%
Notes, due date		Jun. 01, 2021
Notes redemption price			100.00%
Notes issued, fair value				$ 96,300,000		$ 96,300,000
Contractual coupon interest					100,000		1,300,000
Accretion of the debt discount					500,000		2,400,000
Amortization of the debt issuance costs					$ 100,000		$ 400,000
Effective interest rate on the Notes				6.60%		6.60%
Repayment Of Senior Debt		$ 92,300,000	$ 23,000,000.0
Accrued interest		$ 300,000
Total consideration on conversion	$ 253,300,000
Face amount of remaining convertible debt			60,800,000
Equity component of converted debt	$ 163,600,000		38,300,000
Loss on extinguishment of debt			$ 1,100,000			$ 4,600,000
2.125% Convertible Senior Notes due 2021 | Common Stock [Member]
Debt Instrument [Line Items]
Conversion of Convertible Securities Stock Issued | shares | shares	1,850,155		466,045
Conversion of Convertible Securities Stock Issued | value	$ 161,000,000.0		$ 37,800,000
2.125% Non Convertible Senior Notes due 2021
Debt Instrument [Line Items]
Effective interest rate on the Notes				3.90%		3.90%
Equity component of converted debt				$ 200,100,000		$ 200,100,000