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Commitments and contingencies - Additional Information (Detail) (USD $)
 6 Months Ended 12 Months Ended
Dec. 31, 2011
Dec. 31, 2013
Property
Dec. 31, 2012
Property
Loss Contingencies [Line Items]      
Aggregate outstanding commitments   $ 1,453,000 $ 1,694,000
Number of properties   23,268 3,644
Company contributions 93,000 181,000 163,000
Purchase Commitment [Member]
      
Loss Contingencies [Line Items]      
Number of properties   536 462
Aggregate purchase price   $ 75,473,000 $ 70,082,000