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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2017
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)
 
2017
 
2016
 
2015
 
Balance as of January 1
 
$
557.7

 
$
571.2

 
$
43.6

 
Integrys subsidiaries
 
—

 
—

 
491.0

 
Accretion
 
27.5

 
28.3

 
14.5

 
Additions and revisions to estimated cash flows
 
26.5

(1) 
—

 
35.5

(2) 
Liabilities settled
 
(38.0
)
 
(41.8
)
 
(13.4
)
 
Balance as of December 31
 
$
573.7

 
$
557.7

 
$
571.2

 


(1) 
AROs increased $20.5 million in 2017 due to revisions made to estimated cash flows primarily for changes in the weighted average cost to retire natural gas distribution pipe at PGL and NSG. In addition, an ARO of $5.5 million was recorded related to the removal and dismantlement of WE's Rothschild Biomass Plant.

(2) 
During 2015, an ARO of $16.1 million was recorded for fly-ash landfills located at generation facilities owned by WE and WPS. An ARO of $9.0 million was also recorded during 2015 for the Hazardous and Solid Waste Management System; Disposal of Coal Combustion Residuals from Electric Utilities rule passed by the EPA in April 2015. In addition, AROs increased $10.4 million in 2015 due to revisions made to estimated cash flows primarily for changes in the weighted average cost to retire natural gas distribution pipe at PGL and NSG.