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Employee Benefit Plans - Significant Assumptions Used to Calculate Fair Market Values (Detail)
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12 Months Ended | ||
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Dec. 31, 2011
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Dec. 31, 2010
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Dec. 31, 2009
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| Assumptions used | |||
| Risk-free interest rate | 3.10% | 3.23% | 2.90% |
| Dividend yield | 4.00% | 4.00% | 3.50% |
| Volatility factor | 33.40% | 33.80% | 32.90% |
| Expected life (years) | 6 years 10 months 13 days | 6 years 7 months 24 days | 6 years 6 months 11 days |
| Ranges used | |||
| Risk-free interest rate | 3.10% | 3.23% | 2.90% |
| Dividend yield | 4.00% | 4.00% | 3.50% |
| Volatility factor | 33.40% | 33.80% | 32.90% |
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Maximum [Member]
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| Assumptions used | |||
| Risk-free interest rate | 3.10% | 3.23% | 2.90% |
| Dividend yield | 4.00% | 4.00% | 3.50% |
| Volatility factor | 33.40% | 33.80% | 32.90% |
| Ranges used | |||
| Risk-free interest rate | 3.10% | 3.23% | 2.90% |
| Dividend yield | 4.00% | 4.00% | 3.50% |
| Volatility factor | 33.40% | 33.80% | 32.90% |
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Minimum [Member]
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| Assumptions used | |||
| Risk-free interest rate | 3.10% | 3.23% | 2.90% |
| Dividend yield | 4.00% | 4.00% | 3.50% |
| Volatility factor | 33.40% | 33.80% | 32.90% |
| Ranges used | |||
| Risk-free interest rate | 3.10% | 3.23% | 2.90% |
| Dividend yield | 4.00% | 4.00% | 3.50% |
| Volatility factor | 33.40% | 33.80% | 32.90% |