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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2015
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
 
2015
 
2014
 
(in millions)
ARO liability at January 1,(1)
$
195.1

 
$
165.1

Accretion
8.7

 
6.7

Additions(2)
3.8

 
17.1

Revisions
17.2

 
33.6

Liabilities related to assets sold
(16.0
)
 
(24.7
)
Liabilities settled
(2.0
)
 
(2.7
)
ARO liability at December 31,
$
206.8

 
$
195.1