Net loss per share - Additional Information (Detail)	3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013 Class A Common Shares [Member]	Dec. 31, 2012 Class A Common Shares [Member]	Sep. 30, 2013 Class B Common Shares [Member]	Feb. 28, 2013 Class B Common Shares [Member]	Dec. 31, 2012 Class B Common Shares [Member]	Dec. 31, 2012 Formation Transactions [Member] Class A Common Shares [Member]	Dec. 31, 2012 Formation Transactions [Member] Class B Common Shares [Member]
Schedule Of Earnings Per Share Basic And Diluted By Common Class [Line Items]
Common shares issue to sponsor			184,856,219	38,663,998	635,075	634,408	667	3,301,000	667
Total weighted average shares	54,263,266	54,263,266