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Shareholders' Equity - Company's Repurchasing Activity by Repurchase Plan (Detail) (USD $)
216 Months Ended 132 Months Ended
Dec. 31, 2011
Non-Discretionary Plan
Dec. 31, 2011
Discretionary Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Cumulative shares repurchased 23,120,426 21,156,206
Average price per share $ 22.61 $ 35.15