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Accident and Workers' Compensation Insurance Liabilities (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2011
Mar. 31, 2009
Accident and Workers' Compensation Insurance Liabilities    
Auto liability retention limit $ 2.0 $ 1.0
Excess Insurance Coverage Limit 55.0  
Workers compensation retention limit 1.0  
Trust fund requirements 0.7  
Trust fund actual deposits 1.3  
Letters of Credit Outstanding, Amount $ 3.1