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Other Long-Term Liabilities (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 1 Months Ended 3 Months Ended
Jun. 30, 2007
Mar. 31, 2012
Dec. 31, 2011
Mar. 31, 2012
Minimum [Member]
Mar. 31, 2012
Maximum [Member]
Other Long Term Liabilities [Line Items]          
Deferred rent associated with lease obligations   $ 10.7 $ 10.3    
Leasehold improvements, long-term liability amortization period, years       5 10
Deferred lease incentives   3.8 4.1    
Sale and lease back of Loudoun, Virginia campus building, sale price 5.8        
Sale and lease back term, years 10        
Gain on sale and lease back of Loudoun, Virginia campus building, before tax 2.8        
Non-current portion of sale lease back gain   $ 1.2 $ 1.3