Net loss per share - Additional Information (Detail)	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013	Jun. 30, 2013 Class A Common Shares [Member]	Dec. 31, 2012 Class A Common Shares [Member]	Jun. 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			129,433,425	38,663,998	635,075	634,408	667	3,301,000	667
Total weighted average shares	35,775,468	54,293,266