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Fair Value of Options Granted Calculated Using Weighted Average Assumptions (Detail) (USD $)
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3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
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Jun. 30, 2012
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Jun. 30, 2011
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Jun. 30, 2012
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Jun. 30, 2011
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| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||||
| Options granted | 58,550 | 46,950 | 58,550 | |
| Weighted average exercise price | $ 47.89 | $ 41.27 | $ 47.89 | |
| Weighted-average grant date fair value | $ 22.30 | $ 19.08 | $ 22.30 | |
| Assumptions: | ||||
| Expected volatility | 45.15% | 47.70% | 45.15% | |
| Expected term (in years) | 6 years | 5 years 10 months 24 days | 6 years | |
| Risk-free interest rate | 2.64% | 1.43% | 2.64% | |
| Expected dividend yield | ||||