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Summary of Significant Accounting Policies (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Activity in allowance for doubtful accounts      
Balance at beginning of year $ 48.1 $ 50.9 $ 55.2
Additions charged to expense 29.7 21.0 23.6
Accounts written-off (32.5) (23.8) (27.9)
Balance at ending of year $ 45.3 $ 48.1 $ 50.9