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Share-based compensation - Schedule of Restricted Share Units, Common Stock Grants and Restricted Stock Activity (Detail) - Restricted Stock Units, Common Stock Grants and Restricted Stock
9 Months Ended
Sep. 30, 2018
$ / shares
shares
Number of units  
Number of units, beginning balance | shares 8,150,905
Number of units, granted | shares 4,565,125
Number of units, vested and delivered | shares (11,462,280)
Number of units, withheld as treasury stock | shares (872,562)
Number of units, vested not delivered | shares 3,766,068
Number of units, forfeitures | shares (304,625)
Number of units, ending balance | shares 3,842,631
Weighted average grant-date fair value  
Weighted average grant-date fair value, beginning balance | $ / shares $ 9.27
Weighted average grant-date fair value, granted | $ / shares 2.63
Weighted average grant-date fair value, vested and delivered | $ / shares 7.63
Weighted average grant-date fair value, withheld as treasury stock | $ / shares 6.05
Weighted average grant-date fair value, vested not delivered | $ / shares 9.85
Weighted average grant-date fair value, forfeitures | $ / shares 4.82
Weighted average grant-date fair value, ending balance | $ / shares $ 7.95