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 ($) shares	Sep. 30, 2019 USD ($) shares	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	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					4,863,000	$ 3,160,000
Transaction costs attributable to equity component				$ 1,621,000	1,621,000
Total consideration on conversion					198,757,000
Equity component of converted debt				38,088,000	38,088,000
Loss on extinguishment of debt				$ (5,650,000)	(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		$ 278,600,000
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				$ 200,000	200,000
Accretion of the debt discount				1,800,000	1,800,000
Amortization of the debt issuance costs				$ 300,000	$ 300,000
Effective interest rate on the Notes				5.10%	5.10%
Notes, carrying value				$ 230,200,000	$ 230,200,000
Fair value of the note				287,900,000	$ 287,900,000
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					7,300,000
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 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	$ 4,500,000	$ 4,500,000
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]
Notes, due date					Jun. 01, 2021
Effective interest rate on the Notes				3.90%	3.90%
Equity component of converted debt				$ 200,100,000	$ 200,100,000