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Financial Instruments (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
9 Months Ended
Sep. 30, 2011
Dec. 31, 2010
Schedule of Investments [Line Items]  
Cash and cash equivalents of not-for-profit schools with restrictions$ 61.1$ 58.5
Years ARS auction events have been failing2 
Letters of credit outstanding under U.S. Credit Agreement6.9 
Letters of credit total credit availability under U.S. Credit Agreement185.0 
Letters of credit remaining credit availability under U.S. Credit Agreement178.1 
Municipal Bonds [Member]
  
Schedule of Investments [Line Items]  
Cumulative unrealized losses on municipal bonds$ 0.7