Stock-Based Compensation - Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Restricted Stock and RSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Recognized compensation cost	$ 3.3	$ 0.4
Unrecognized compensation cost related to non-vested restricted stock	9.9
Unrecognized compensation cost, period	2 years 9 months 15 days
Restricted Stock and RSUs [Member] \| Service Providers [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Recognized compensation cost	2.5
Restricted Stock and RSUs [Member] \| Consultant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Recognized compensation cost	1.9
Restricted Stock and RSUs [Member] \| Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Recognized compensation cost	2.7	0.1
Restricted Stock and RSUs [Member] \| General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Recognized compensation cost	0.6	0.3
Options to purchase common stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Recognized compensation cost	2.1
Unrecognized compensation cost, period	3 years 6 months 26 days
Weighted average grant date fair value of options granted	$ 48.73
Unrecognized compensation cost related to stock options	46.3
Options to purchase common stock [Member] \| Service Providers [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Recognized compensation cost	0.3
Options to purchase common stock [Member] \| Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Recognized compensation cost	0.9
Options to purchase common stock [Member] \| General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Recognized compensation cost	$ 1.2

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 14.F)

-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (h)(1)(i)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 14

-Section F

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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (i)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

+ References

No definition available.

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- Definition

Unrecognized cost of unvested options awarded to employees as compensation.

+ References

No definition available.

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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (d)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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