Shareholders' equity - Additional Information (Detail)			1 Months Ended			9 Months Ended				9 Months Ended
	Sep. 30, 2013 2013 Offering [Member]	Sep. 30, 2013 2012 Offering [Member]	Aug. 31, 2013 Class A Common Shares [Member]	Sep. 30, 2013 Class A Common Shares [Member] Sponsor [Member]	Dec. 31, 2012 Class A Common Shares [Member] Sponsor [Member]	Sep. 30, 2013 Class B Common Shares [Member] Vote	Sep. 30, 2013 Class B Common Shares [Member] Sponsor [Member]	Sep. 30, 2013 Class A Units [Member] Sponsor [Member]	Dec. 31, 2012 Class A Units [Member] Sponsor [Member]	Sep. 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 in connection with the IPO			55,422,794
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				3.70%	8.50%					93.10%	99.90%
Units outstanding								13,787,292	32,668	199,278,586	38,697,333
Stock split conversion ratio										1