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Summary of Changes in Stock Options Outstanding (Detail) (USD $)
In Millions, except Per Share data, unless otherwise specified
6 Months Ended
Jun. 30, 2011
Year
Investment
Options  
Beginning Balance 2.6
Expired  
Exercised (1.3) [1]
Ending Balance 1.3
Per Share Weighted Average Exercise Price  
Beginning Balance $ 11.13
Expired  
Exercised $ 11.50 [1]
Ending Balance $ 10.79
Weighted Average Remaining Contractual Term  
Beginning Balance 2.59
Ending Balance 1.80
Aggregate Intrinsic Value  
Beginning Balance $ 58.2
Ending Balance $ 40.5
[1] Options exercised during the six months ended June 30, 2011 had an intrinsic value of $34.0 million.