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Debt instrument origination fee percentage.

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The reference rate for the variable rate of the debt instrument, such as LIBOR or the US Treasury rate and the maturity of the reference rate used, such as three months or six months LIBOR.

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Face (par) amount of debt instrument at time of issuance.

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Amount of the fee that accompanies borrowing money under the debt instrument.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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Amount, after accumulated amortization, of debt discount.

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Amount, before accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

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Amount of the cost of borrowed funds accounted for as interest expense for debt.

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Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

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Date the first payment under the line of credit must be paid, in CCYY-MM-DD format.

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Amount of the required periodic payment applied to principal.

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Carrying value as of the balance sheet date of loans payable (with maturities initially due after one year or beyond the operating cycle if longer), excluding current portion.

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Indebtedness - Additional Information (Detail)	1 Months Ended			3 Months Ended		9 Months Ended
Indebtedness - Additional Information (Detail)	Sep. 30, 2018 USD ($)	Nov. 30, 2017 USD ($) shares $ / shares	Jul. 31, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Debt extinguishment loss						$ (2,322,000)
Long-term debt	$ 415,446,000			$ 415,446,000		415,446,000		$ 424,876,000
2024 Convertible Notes [Member]
Debt Instrument [Line Items]
Long-term debt	415,400,000			415,400,000		415,400,000
Interest expense related to debt facilities				10,700,000	$ 800,000	26,500,000	$ 1,200,000
2024 Convertible Notes [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	570,000,000	$ 570,000,000		570,000,000		$ 570,000,000		570,000,000
Debt instrument, maturity date		Nov. 15, 2024
Debt instrument, interest rate per annum		1.50%
Debt instrument, payment frequency						The 2024 Notes were issued at face value and bear interest at the rate of 1.50% per annum, payable semi-annually in cash on each May 15 and November 15, commencing on May 15, 2018. There are no principal payments due prior to maturity.
Debt instrument, conversion rate		0.013621
Debt instrument, conversion price \| $ / shares		$ 73.42
Debt discount	144,999,000	$ 171,800,000		144,999,000		$ 144,999,000		$ 158,890,000
Proceeds for issuance of notes recorded to equity component		161,200,000
Debt issuance costs		$ 10,600,000
Effective interest rate percentage								6.90%
2024 Convertible Notes [Member] \| Level 1 [Member]
Debt Instrument [Line Items]
Debt fair value	1,300,000,000			$ 1,300,000,000		$ 1,300,000,000
2024 Convertible Notes [Member] \| Common Stock [Member]
Debt Instrument [Line Items]
Debt instrument, convertible into shares \| shares		7,763,552
MidCap Financial Amended and Restated Credit and Security Agreement [Member] \| July 2017 Term Loan [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate per annum			6.25%
Current borrowing capacity			$ 60,000,000
Variable interest rate description			one-month London Interbank Offered Rate
Monthly principal payments commencement date			Jul. 01, 2018
Periodic principal payment frequency			monthly
Monthly principal payments			$ 800,000
Midcap Financial Revolving Credit and Security Agreement [Member] \| Revolving Loan [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate per annum			3.95%
Current borrowing capacity			$ 40,000,000
Variable interest rate description						one-month LIBOR
Line of credit facility additional tranche that may be increased			20,000,000
Origination fee			$ 200,000
Origination fee, percentage			0.50%
MidCap Financial Amended and Restated Credit and Security Agreement and Revolving Credit and Security Agreement [Member]
Debt Instrument [Line Items]
Agreement termination date						2018-09
MidCap Financial Amended and Restated Credit and Security Agreement and Revolving Credit and Security Agreement [Member] \| July 2017 Term Loan [Member]
Debt Instrument [Line Items]
Debt extinguishment loss	$ (2,300,000)