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Asset Retirement Obligations (Tables)
 9 Months Ended
Sep. 30, 2017
Asset Retirement Obligation [Abstract]  
Asset retirement obligations rollforward [Table Text Block]
The following is a reconciliation of the changes in the Company's ARO for the period specified below:
 
Asset Retirement Obligations
 
2017
 
(in millions)
ARO liability at January 1,
$
231.6

Accretion
6.0

Additions
6.2

Revisions
0.2

Liabilities related to assets sold(1)
(40.8
)
Liabilities settled
(9.0
)
ARO liability at September 30,
$
194.2


____________________________
(1)  
Liabilities related to assets sold for the nine months ended September 30, 2017, includes $34.9 million related to the Pinedale Divestiture (refer to Note 2 – Acquisitions and Divestitures for more information).