Shareholders' equity - Additional Information (Detail)				1 Months Ended			6 Months Ended		1 Months Ended	6 Months Ended				6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013 2013 Offering [Member]	Jun. 30, 2013 2012 Offering [Member]	Aug. 21, 2013 Class A Common Shares [Member] Subsequent Events [Member]	Jun. 30, 2013 Class A Common Shares [Member] Sponsor [Member]	Dec. 31, 2012 Class A Common Shares [Member] Sponsor [Member]	Jun. 30, 2013 Class A Common Shares [Member] 2013 Offering [Member]	Jun. 30, 2012 Class A Common Shares [Member] 2012 Offering [Member]	Aug. 21, 2013 Class A Common Shares [Member] IPO [Member] Subsequent Events [Member]	Jun. 30, 2013 Class B Common Shares [Member] Vote	Jun. 30, 2013 Class B Common Shares [Member] Sponsor [Member]	Jun. 30, 2013 Class A Units [Member] Sponsor [Member]	Dec. 31, 2012 Class A Units [Member] Sponsor [Member]	Jun. 30, 2013 Class A Units [Member] Operating Partnership [Member]	Dec. 31, 2012 Class A Units [Member] Operating Partnership [Member]
Changes In Equity And Comprehensive Income Line Items [Line Items]
Common shares sold							46,718,750	35,360,898
Board of trustees approval extension period	180 days
Percentage of common shares outstanding							75.00%	75.00%
Common shares sold in connection with the IPO				55,422,794					6,617,647
Common stock issued in connection with investment											635,075
Common shares entitled to vote										50
Voting interest		30.00%	30.00%
Percentage of units outstanding					2.90%	8.50%								90.42%	99.92%
Units outstanding												13,787,292	32,668	143,855,792	38,697,333
Stock split conversion ratio														1