Significant accounting policies - Additional Information (Detail)	6 Months Ended
Jun. 30, 2013
Minimum [Member]
Property Subject to or Available for Operating Lease [Line Items]
Acquired leases term	6 months
Maximum [Member]
Property Subject to or Available for Operating Lease [Line Items]
Acquired leases term	2 years

Term of the lessor's leasing arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	us-gaap_LessorLeasingArrangementsOperatingLeasesTermOfContract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration