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Stock Plans - Additional Information (Detail) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended 3 Months Ended 12 Months Ended 3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Mar. 31, 2013
Equity incentive plan restricted stock [Member]
Dec. 31, 2012
Equity incentive plan restricted stock [Member]
Mar. 31, 2013
Restricted stock awards for certain management [Member]
Dec. 31, 2011
Restricted stock awards for certain management [Member]
Mar. 31, 2013
Restricted stock awards for certain management [Member]
Minimum [Member]
Mar. 31, 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     575,659 574,933        
Compensation cost on stock-based compensation plans $ 2.7 $ 2.4