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Debt Credit Agreement (Details) (USD $)
12 Months Ended 12 Months Ended 12 Months Ended
Dec. 31, 2011
Quarters
Dec. 31, 2010
Dec. 31, 2009
Dec. 31, 2011
Letter of Credit [Member]
Dec. 31, 2011
Senior Notes [Member]
6.25% Interest Rate [Member]
Feb. 03, 2012
New Contract [Member]
Dec. 31, 2011
New Contract [Member]
Feb. 03, 2012
New Contract [Member]
Letter of Credit [Member]
Feb. 03, 2012
Unitholder Distributions [Member]
New Contract [Member]
Dec. 31, 2011
Minimum [Member]
Dec. 31, 2011
Minimum [Member]
Option (a) [Member]
Dec. 31, 2011
Minimum [Member]
Option (b) [Member]
Dec. 31, 2011
Maximum [Member]
Dec. 31, 2011
Maximum [Member]
Option (a) [Member]
Dec. 31, 2011
Maximum [Member]
Option (b) [Member]
Debt Instrument [Line Items]                              
Line of Credit Facility, Maximum Borrowing Capacity       $ 275,000,000   $ 375,000,000 $ 275,000,000 $ 50,000,000 $ 30,000,000            
Proceeds from Lines of Credit 118,000,000 66,000,000 239,000,000 118,000,000                      
Repayments of Lines of Credit 77,000,000 113,000,000 233,000,000 77,000,000                      
Proceeds from (Repayments of) Lines of Credit       41,000,000                      
Line of Credit Facility, Amount Outstanding       $ 200,000,000                      
Debt Instrument, Interest Rate, Stated Percentage         6.25%                    
Debt Instrument, Basis Spread on Variable Rate                     1.00% 2.00%   2.00% 3.00%
Line of Credit Facility, Commitment Fee Percentage                   0.375%     0.50%    
EBITDA, Based on Number of Most Recently Completed Fiscal Quarters 4