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Share-based compensation - Summary of Outstanding Restricted Shares, Phantom Share Units, SARs and Restricted Stock Units (Detail) - $ / shares
12 Months Ended
Mar. 27, 2013
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Granted       1,664,457
Restricted shares [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Beginning balance, Number of shares   43,936 48,880 63,653
Granted   51,368 43,936 54,990
Vested   (45,924) (48,880) (65,578)
Cancelled   (4,433) 0 (4,185)
Ending balance, Number of shares   44,947 43,936 48,880
Beginning balance, W.A. grant date FV   $ 22.57 $ 17.01 $ 14.17
Granted   18.39 22.57 17.01
Vested   22.39 17.01 14.25
Cancelled   18.39   17.01
Ending balance, W.A. grant date FV   $ 18.39 $ 22.57 $ 17.01
Phantom share units [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Beginning balance, Number of shares   707,000 657,000 562,000
Granted   100,000 70,000 95,000
Exchanged   (110,000) (20,000)  
Cancelled   (49,999)    
Ending balance, Number of shares   647,001 707,000 657,000
Beginning balance, W.A. grant date FV   $ 14.77 $ 14.02 $ 13.13
Granted   18.24 23.04 19.30
Exchanged   16.21 19.00  
Cancelled   19.66    
Ending balance, W.A. grant date FV   $ 14.73 $ 14.77 $ 14.02
Stock appreciation rights (SARs) [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Beginning balance, Number of shares   5,879,416 7,072,945 5,674,148
Granted 1,664,457     1,664,457
Exercised     (1,193,529) (241,906)
Cancelled   (2,605)   (23,754)
Ending balance, Number of shares   5,876,811 5,879,416 7,072,945
Beginning balance, W.A. grant date FV   $ 2.30 $ 2.32 $ 2.03
Granted       3.51
Exercised     2.42 3.65
Cancelled   3.65   3.51
Ending balance, W.A. grant date FV   $ 2.30 $ 2.30 $ 2.32
Restricted stock units [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Beginning balance, Number of shares   35,076    
Granted   38,142 72,314  
Vested   (35,195) (37,238)  
Cancelled   (5,195)    
Ending balance, Number of shares   32,828 35,076  
Beginning balance, W.A. grant date FV   $ 23.03    
Granted   20.21 $ 23.03  
Vested   22.01 23.03  
Cancelled   21.86    
Ending balance, W.A. grant date FV   $ 21.03 $ 23.03