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Stock Compensation Plans - Purchased Restricted Stock Plan - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
PRS [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares issued 58,629 52,577 99,091
Value of shares purchased by employees $ 7.0 $ 6.2 $ 9.7
Shares purchased by employees 29,315 33,600 49,545
Total fair value of vested $ 10.2    
Unrecognized compensation cost related to non-vested stock options and SSAR, RSU, PRS and Cash RSU awards granted $ 8.3    
Unrecognized compensation cost related to non-vested stock options and SSAR, RSU, PRS and Cash RSU awards granted, weighted average period, in years 1 year 9 months 24 days    
PRSUs [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares issued 183,000 14,622  
Value of shares purchased by employees   $ 1.7  
Total fair value of vested $ 22.8