Quarterly report pursuant to Section 13 or 15(d)

Share-Based Compensation - Additional Information (Detail)

v3.8.0.1
Share-Based Compensation - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Jun. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share-based compensation expense $ 9.9   $ 9.8 $ 32.6 $ 30.3
Unrecognized compensation cost on non-vested stock awards 55.5     $ 55.5  
Unrecognized compensation cost on non-vested stock awards, weighted-average period of recognition       1 year 6 months  
Total vesting date fair value of stock units $ 0.1   $ 0.2 $ 1.9 $ 2.0
SARs [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Weighted-average grant date fair value of SARs granted $ 31.31   $ 32.06 $ 28.36 $ 29.49
Total intrinsic value of awards exercised for SARs $ 3.0   $ 17.9 $ 100.0 $ 32.0
Performance conditions [Member] | Chief Executive Officer [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Performance stock unit awards vesting date Dec. 31, 2019 Dec. 31, 2019      
Performance conditions [Member] | Minimum [Member] | Chief Executive Officer [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Performance stock unit awards vesting percentage 0.00% 0.00%      
Performance conditions [Member] | Maximum [Member] | Chief Executive Officer [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Performance stock unit awards vesting percentage 200.00% 200.00%