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Share-based payments - Schedule of Unvested Restricted Share Activity (Detail) - Restricted Stock Units [Member]
 3 Months Ended
Mar. 31, 2017
$ / shares
shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]  
Number of units, Beginning Balance | shares 12,407,029
Number of units, Vested and delivered | shares (677,545)
Number of units, Withheld as treasury stock | shares (44,796) [1]
Number of units, Vested not delivered | shares (2,238,254) [2]
Number of units, Forfeitures | shares (77,250)
Number of units, Ending Balance | shares 9,369,184
Weighted average grant-date fair value, Beginning Balance | $ / shares $ 8.40
Weighted average grant-date fair value, Vested and delivered | $ / shares 7.93
Weighted average grant-date fair value, Withheld as treasury stock | $ / shares 12.08 [1]
Weighted average grant-date fair value, Vested not delivered | $ / shares 9.71 [2]
Weighted average grant-date fair value, Forfeitures | $ / shares 5.39
Weighted average grant-date fair value, Ending Balance | $ / shares $ 8.13
[1] As discussed in Note 8, the increase in treasury stock was due to shares withheld to pay statutory withholding taxes upon the vesting of RSUs during the three months ended March 31, 2017.
[2] Vested not delivered represent the vested RSUs with deferred delivery at a future time. As of March 31, 2017, the cumulative shares of RSUs included in “vested not delivered” above were 6,345,921.