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Summary of Changes in Stock Options Outstanding (Detail) (USD $)
In Millions, except Per Share data, unless otherwise specified
9 Months Ended
Sep. 30, 2011
Year
Entity
Investment
Options 
Beginning Balance2.6
Expired 
Exercised(1.3)[1]
Ending Balance1.3
Per Share Weighted Average Exercise Price 
Beginning Balance$ 11.13
Expired 
Exercised$ 11.41[1]
Ending Balance$ 10.84
Weighted Average Remaining Contractual Term 
Beginning Balance2.59
Ending Balance1.60
Aggregate Intrinsic Value 
Beginning Balance$ 58.2
Ending Balance$ 31.4
[1]Options exercised during the nine months ended September 30, 2011 had an intrinsic value of $35.7 million.