Asset Retirement Obligations (Tables)

6 Months Ended

Jun. 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	4.3
Additions	3.3
Revisions	0.2
Liabilities settled	(7.7		)
ARO liability at June 30,	$	231.7