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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2016
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
 
2016
 
2015
 
(in millions)
ARO liability at January 1,
$
206.8

 
$
195.1

Accretion
8.9

 
8.7

Additions(1)
17.0

 
3.8

Revisions
6.5

 
17.2

Liabilities related to assets sold
—

 
(16.0
)
Liabilities settled
(7.6
)
 
(2.0
)
ARO liability at December 31,
$
231.6

 
$
206.8