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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2017
Asset Retirement Obligation [Abstract]  
Schedule of Change in Asset Retirement Obligation [Table Text Block]
The following is a reconciliation of the changes in the Company's ARO for the periods specified below:
 
Asset Retirement Obligations
 
2017
 
2016
 
(in millions)
ARO liability at January 1,
$
231.6

 
$
206.8

Accretion
7.7

 
8.9

Additions(1)
23.5

 
17.0

Revisions
8.5

 
6.5

Liabilities related to assets sold(2)
(34.9
)
 
—

Liabilities settled
(22.3
)
 
(7.6
)
ARO liability at December 31,
$
214.1

 
$
231.6