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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2011
Asset Retirement Obligations (Tables) [Abstract]  
Change in Asset Retirement Obligations
The following table presents the change in our AROs during 2011 and 2010:

 
2011
 
2010
 
(Millions of Dollars)
 
 
 
 
Balance as of January 1
$
52.6

 
$
57.9

Liabilities Incurred
0.6

 
—

Liabilities Settled
(2.2
)
 
(2.5
)
Accretion
3.0

 
3.1

Cash Flow Revisions
1.5

 
(5.9
)
Balance as of December 31
$
55.5

 
$
52.6