Debt - Summary of Carrying Value of Convertible Notes (Detail) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Gross proceeds		$ 201,250
Amortization of debt discount and debt issuance costs	$ 3,811	$ 1,190
Carrying value 2024 Convertible Notes	140,902		$ 137,107
2024 Convertible Notes [Member]
Debt Instrument [Line Items]
Gross proceeds	201,250
Gross proceeds, Portion of proceeds allocated to equity component (additional paid-in capital)	(64,541)
Debt issuance costs	(6,470)
Debt issuance costs, Portion of issuance costs allocated to equity component (additional paid-in capital)	2,075
Amortization of debt discount and debt issuance costs	8,588
Carrying value 2024 Convertible Notes	$ 140,902

Debt issuance costs portion allocated to equity.

No definition available.

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- Definition

Gross convertible debt portion allocated to equity.

+ References

No definition available.

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- Definition

Proceeds from convertible debt gross.

+ References

No definition available.

+ Details

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- Definition

Amount of amortization expense attributable to debt discount (premium) and debt issuance costs.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28555-108399

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- Definition

Including the current and noncurrent portions, carrying value as of the balance sheet date of a written promise to pay a note, initially due after one year or beyond the operating cycle if longer, which can be exchanged for a specified amount of one or more securities (typically common stock), at the option of the issuer or the holder.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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- Definition

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.

+ References

No definition available.

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
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