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Share-based compensation - Schedule of Restricted Share Units, Common Stock Grants and Restricted Stock Activity - Phantom (Detail) - shares
3 Months Ended 9 Months Ended
Sep. 30, 2018
Mar. 31, 2018
Sep. 30, 2018
Restricted Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Outstanding shares withheld as treasury stock     1,225,085
Restricted Stock Units, Common Stock Grants and Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share units, granted     4,565,125
Share vested not delivered     11,462,280
Shares vested but deferred in prior periods     6,278,318
Outstanding shares withheld as treasury stock     872,562
Restricted Stock Units or Common Stock Grants      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Decrease in vested and unissued shares in prior period     3,766,068
Weighted average number of shares outstanding, Diluted (in shares) 2,909,917   2,909,917
Spin-off Grants | Restricted Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share units, granted     304,000
Share vested not delivered   304,000 2,041,000
Award vesting period     3 years