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Share-based incentives - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended
May. 31, 2015
Jan. 31, 2015
May. 31, 2014
Jan. 31, 2014
Time-Based Restricted Stock        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award vesting period   3 years   3 years
Performance-Based Shares        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award vesting period   3 years   3 years
Time Based Restricted Stock And Performance Based Shares        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Aggregate fair values at grant date   $ 4,200   $ 4,000
Share-based incentives, shares issued   113,000   124,000
Average grant price (in usd per share)   $ 37.04   $ 32.22
Director        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares granted and immediately vested in relation to the annual director retainer compensation (in shares) 18,184   22,179  
Shares granted and immediately vested in relation to the annual director retainer compensation, fair value $ 660   $ 670