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Incentive Plans (Details) (USD $)
In Millions, except Share data in Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Summary of activity in outstanding grants      
Outstanding grants, beginning of the period 753,868    
Grants during the period 279,078    
Grants vested and paid during the period (162,186)    
Forfeitures during the period (146)    
Outstanding grants, end of the period 870,344 753,868  
Incentive Plans (Textuals) [Abstract]      
Vesting period of Grants in years 4 years    
Partnership's historical distribution rate 6.61%    
Expenses related to Incentive Plan to be reimbursed to general partner $ 7.4 $ 9.0 $ 7.0
Payments made in connection with Long-Term Incentive Plan 5.7 3.2 2.9
Unaccrued cost associated with outstanding grants and related DERs $ 11.1    
Market value of common units under the incentive plan Average closing price over the last 20 trading days prior to the vesting date    
Grant date fair value $ 42.93 $ 29.42 $ 31.01
Maximum [Member]
      
Unit Based Compensation Arrangement [Line Items]      
Risk free interest rate 0.37%    
Volatility rate 40.14%    
Minimum [Member]
      
Unit Based Compensation Arrangement [Line Items]      
Risk free interest rate 0.13%    
Volatility rate 35.66%