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Other Long-Term Liabilities (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 1 Months Ended 12 Months Ended
Jun. 30, 2007
Dec. 31, 2011
Dec. 31, 2010
Deferred rent associated with lease obligations   $ 10.3 $ 7.9
Leasehold improvements reimbursed by lessors   2.0 0.7
Deferred lease incentives   4.1 3.2
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   $ 1.3 $ 1.5
Minimum [Member]
      
Leasehold improvements, long-term liability amortization period, years   5  
Maximum [Member]
      
Leasehold improvements, long-term liability amortization period, years   10