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Financial Instruments - Additional Information (Detail) (USD $)
 3 Months Ended 9 Months Ended
Sep. 30, 2012
Y
Sep. 30, 2011
Sep. 30, 2012
Y
Sep. 30, 2011
Dec. 31, 2011
Schedule of Investments [Line Items]          
Period cash equivalents and short-term investments have been in continuous unrealized loss position, years, maximum 1   1    
Cash and cash equivalents of not-for-profit schools with restrictions $ 70,200,000   $ 70,200,000   $ 74,500,000
Cumulative unrealized losses on municipal bonds (206,000) (6,000) (326,000) 40,000  
ARS auction events have been failing, years, minimum     3    
Letters of credit outstanding under U.S. Credit Agreement 6,200,000   6,200,000    
Letters of credit total credit availability under U.S. Credit Agreement 185,000,000   185,000,000    
Letters of credit remaining credit availability under U.S. Credit Agreement 178,800,000   178,800,000    
U.S. Government Agencies [Member]
          
Schedule of Investments [Line Items]          
Period debt obligations mature, years, maximum     1    
Municipal Bonds [Member]
          
Schedule of Investments [Line Items]          
ARS stated terms to maturity, years, minimum     1    
Cumulative unrealized losses on municipal bonds     $ 500,000