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Stock Plans - Additional Information (Detail) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended 6 Months Ended 6 Months Ended 12 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Equity Incentive Plan Restricted Stock [Member]
Dec. 31, 2012
Equity Incentive Plan Restricted Stock [Member]
Jun. 30, 2013
Restricted Stock Awards for Certain Management [Member]
Dec. 31, 2011
Restricted Stock Awards for Certain Management [Member]
Jun. 30, 2013
Restricted Stock Awards for Certain Management [Member]
Minimum [Member]
Jun. 30, 2013
Restricted Stock Awards for Certain Management [Member]
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                    
Restricted stock award granted by Company             37,000 67,400    
Vesting period from the date of grant             May vest in 2016 May vest in 2014    
Performance period             3 years      
Weighted-average grant-date fair value of restricted stock awards granted             $ 45.01 $ 44.39    
Risk-free rate of weighted-average grant date fair value             0.40%      
Volatility rate of weighted-average grant date fair value                 23.00% 64.00%
Expected life assumption for weighted average grant date fair value             3 years      
Restricted stock award, outstanding         524,596 574,933        
Compensation cost on stock-based compensation plans $ 2.0 $ 2.9 $ 4.6 $ 5.3