Commitments and contingencies - Additional Information (Detail) (USD $)	Jun. 30, 2013 Property	Dec. 31, 2012 Property
Loss Contingencies [Line Items]
Aggregate outstanding commitments	$ 9,382,000	$ 1,694,000
Number of properties	18,326	3,644
Purchase Commitment [Member]
Loss Contingencies [Line Items]
Number of properties	1,152	462
Aggregate purchase price	$ 167,318,000	$ 70,082,000