Significant accounting policies - Additional Information (Detail)	9 Months Ended
Sep. 30, 2013
Accounting Policies [Abstract]
Acquired leases term	Less than one year

A general description of the nature of the existing leasing arrangements of a lessor for all operating leases including, but not limited to: (1) guarantees or indemnities; (2) restrictions imposed by lease arrangements; (3) unusual provisions or conditions; (4) contingent rentals; and (5) lease expiration dates.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 10

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6452767&loc=d3e37045-112695

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration