Convertible Senior Notes - Additional Information (Detail)	1 Months Ended
	Sep. 30, 2014 USD ($) $ / shares	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Interest rate of debt, stated percentage		0.75%
Net carrying amount		$ 326,512,000	$ 318,957,000
Aggregate amount paid for Note Hedges	$ 63,900,000
Proceeds from sale of warrants	34,500,000
Liability component
Debt Instrument [Line Items]
Net carrying amount		326,512,000	318,957,000
Equity component
Debt Instrument [Line Items]
Net carrying amount		62,061,000	62,061,000
Equity component, gross
Debt Instrument [Line Items]
Net carrying amount	63,600,000	$ 63,643,000	$ 63,643,000
0.75% Convertible Senior Notes Due 2019
Debt Instrument [Line Items]
Debt instrument, principal amount	$ 345,000,000
Interest rate of debt, stated percentage	0.75%	0.75%
Net proceeds from issuance of debt	$ 336,300,000
Net proceeds used to purchase Note Hedges	$ 29,400,000
Conversion rate, number of share per $1,000 principal amount	18.9667
Conversion rate, principal amount of Notes	$ 1,000
Initial conversion price (in USD per share) | $ / shares	$ 52.72
Effective interest rate percentage	4.98%	4.98%
Deferred tax liability	$ 23,700,000
0.75% Convertible Senior Notes Due 2019 | Liability component
Debt Instrument [Line Items]
Debt issuance costs	7,000,000
0.75% Convertible Senior Notes Due 2019 | Liability component, gross
Debt Instrument [Line Items]
Net carrying amount	281,400,000
0.75% Convertible Senior Notes Due 2019 | Equity component
Debt Instrument [Line Items]
Debt issuance costs	$ 1,600,000
0.75% Convertible Senior Notes Due 2019 | Debt issuance costs
Debt Instrument [Line Items]
Effective interest rate percentage	5.46%	5.46%
Warrant
Debt Instrument [Line Items]
Common stock strike price per share | $ / shares	$ 68.86