401(k) Savings plan - Additional Information (Detail) (401 (k) [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014
401 (k) [Member]
Defined Benefit Plan Disclosure [Line Items]
Contributions to the 401(k) Plan	$ 0

The increase in the fair value of plan assets from contributions made by the employer.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (b)(3)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920

Name:	us-gaap_DefinedBenefitPlanContributionsByEmployer
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration