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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% |  |

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