Organization and operations - Additional Information (Detail) (USD $)			1 Months Ended		9 Months Ended	1 Months Ended				1 Months Ended
	Sep. 30, 2013 Property State	Dec. 31, 2012 Property	Dec. 31, 2012 2012 Offering [Member] Property	Mar. 31, 2013 2013 Offering [Member]	Sep. 30, 2013 2013 Offering [Member]	Aug. 31, 2013 IPO [Member]	Aug. 31, 2013 2013 Concurrent Private Placements [Member]	Jun. 10, 2013 Operating Partnership [Member] Series D Convertible Units [Member]	Jun. 10, 2013 Operating Partnership [Member] Series E Convertible Units [Member]	Dec. 31, 2012 Sponsor [Member] 2012 Offering [Member]
Schedule Of Description Of Business [Line Items]
Number of properties	21,267	3,644	367
Number of states	22
Proceeds from offering before fees			$ 530,413,000	$ 747,500,000		$ 811,764,000	$ 75,000,000			$ 5,307,000
Fees associated with the sale of stock			40,928,000	44,003,000	85,984,000
Underwriting discounts and offering costs						$ 41,981,000
Issuance of units								4,375,000	4,375,000