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Share-based incentives - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 3 Months Ended
Jan. 31, 2015
Jan. 31, 2014
Jun. 30, 2015
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  
Restricted Common Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based incentives, value of shares issued $ 4.2 $ 4.0 $ 0.2
Share-based incentives, shares issued 113,000 124,000 5,000
Average grant price $ 37.04 $ 32.22 $ 39.34