Commitments and contingencies - Additional Information (Detail) (USD $)	Sep. 30, 2013 Property	Dec. 31, 2012 Property
Loss Contingencies [Line Items]
Aggregate outstanding commitments	$ 4,612,000	$ 1,694,000
Number of properties	21,267	3,644
Purchase Commitment [Member]
Loss Contingencies [Line Items]
Number of properties	416	462
Aggregate purchase price	$ 57,573,000	$ 70,082,000