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Additional Cash Flow Information - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
6 Months Ended 12 Months Ended
Dec. 31, 2010
Dec. 31, 2011
Jun. 30, 2010
Jun. 30, 2009
Other Significant Noncash Transactions [Line Items]        
Gain (Loss) on mark-to-market of listed investments $ 28.8 $ (26.4) $ 24.9 $ (122.4)