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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2018
Asset Retirement Obligation Disclosure [Abstract]  
Schedule of changes to asset retirement obligations
The following table shows changes to our AROs during the years ended December 31:
(in millions)
 
2018
 
2017
 
2016
Balance as of January 1
 
$
573.7

 
$
557.7

 
$
571.2

Accretion
 
28.0

 
27.5

 
28.3

Additions and revisions to estimated cash flows
 
(104.5
)
(1) 
26.5

(2) 
—

Liabilities settled
 
(35.8
)
 
(38.0
)
 
(41.8
)
Balance as of December 31
 
$
461.4

 
$
573.7

 
$
557.7



(1) 
AROs decreased $127.3 million in 2018 due to revisions made to estimated cash flows primarily for changes in the cost to retire natural gas distribution pipe at PGL. Also in 2018, AROs increased $10.7 million as a result of revisions made to estimated cash flows for the abatement of asbestos at WPS's Pulliam power plant, and a $10.9 million ARO was recorded for the legal requirement to dismantle, at retirement, the wind generation projects known as Forward Wind Energy Center and Bishop Hill III. See Note 2, Acquisitions, for more information on Forward Wind Energy Center and Bishop Hill III.

(2) 
AROs increased $20.5 million in 2017 due to revisions made to estimated cash flows primarily for changes in the cost to retire natural gas distribution pipe at PGL and NSG.