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Share Based Compensation - Weighted Average Assumptions Used in Calculation of Fair Value for Service Condition Awards (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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Market and performance condition awards [Member]
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| Share Based Payment Award Stock Options Valuation Assumptions [Line Items] | |||
| Expected volatility | 44.00% | ||
| Dividends yield | 1.40% | ||
| Expected term | 5 years 2 months 12 days | ||
| Risk-free interest rate | 1.20% | ||
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SARs [Member]
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| Share Based Payment Award Stock Options Valuation Assumptions [Line Items] | |||
| Expected volatility | 48.40% | 46.60% | 48.30% |
| Dividends yield | 2.70% | 1.50% | 1.80% |
| Expected term | 5 years 3 months 18 days | 6 years 2 months 12 days | 6 years 4 months 24 days |
| Risk-free interest rate | 0.70% | 1.90% | 2.40% |
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Independent Director's SARs [Member]
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| Share Based Payment Award Stock Options Valuation Assumptions [Line Items] | |||
| Expected volatility | 52.10% | 49.40% | 48.20% |
| Dividends yield | 2.70% | 1.50% | 1.70% |
| Expected term | 3 years 9 months 18 days | 3 years 9 months 18 days | 3 years 9 months 18 days |
| Risk-free interest rate | 0.40% | 1.00% | 1.30% |