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Assumptions Used to Value Options Granted and Their Expected Lives (Detail)
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12 Months Ended | ||
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Dec. 31, 2012
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Dec. 31, 2011
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Dec. 31, 2010
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| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | |||
| Weighted-average expected dividend yield | 0.99% | 0.85% | 0.71% |
| Weighted-average expected volatility | 31.00% | 36.00% | 35.00% |
| Weighted-average risk-free interest rate | 1.80% | 2.10% | 2.80% |
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Minimum
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| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | |||
| Expected life (in years) | 3 years | 0 years | 3 years |
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Maximum
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| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | |||
| Expected life (in years) | 6 years 8 months 12 days | 6 years 3 months 18 days | 5 years 10 months 24 days |