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ASSET RETIREMENT OBLIGATIONS
 12 Months Ended
Dec. 31, 2014
Asset Retirement Obligation Disclosure [Abstract]  
ASSET RETIREMENT OBLIGATIONS
Asset Retirement Obligations

The utility segments have asset retirement obligations primarily related to removal of natural gas distribution mains and service pipes (including asbestos and PCBs); asbestos abatement at certain generation facilities, office buildings, and service centers; dismantling wind generation projects; disposal of PCB-contaminated transformers; closure of fly-ash landfills at certain generation facilities; and removal of above ground storage tanks. The utilities establish regulatory assets and liabilities to record the differences between ongoing expense recognition under the asset retirement obligation accounting rules and the ratemaking practices for retirement costs authorized by the applicable regulators. PDI has asset retirement obligations related to the removal of solar equipment components.

The following table shows changes to our asset retirement obligations through December 31, 2014:
(Millions)
 
Utilities
 
PDI
 
Total
Asset retirement obligations at December 31, 2011
 
$
395.8

 
$
0.5

 
$
396.3

Accretion
 
20.3

 
0.1

 
20.4

Additions and revisions to estimated cash flows
 
(2.3
)
 
1.6

 
(0.7
)
Settlements
 
(5.4
)
 
—

 
(5.4
)
Asset retirement obligations at December 31, 2012
 
408.4

 
2.2

 
410.6

Accretion
 
20.8

 
0.1

 
20.9

Additions and revisions to estimated cash flows
 
70.1

*
0.5

 
70.6

Settlements
 
(11.1
)
 
—

 
(11.1
)
Asset retirement obligations at December 31, 2013
 
488.2

 
2.8

 
491.0

Accretion
 
24.5

 
0.1

 
24.6

Additions and revisions to estimated cash flows
 
(18.3
)
*
0.7

 
(17.6
)
Settlements
 
(17.8
)
 
—

 
(17.8
)
Asset retirement obligations at December 31, 2014
 
$
476.6

 
$
3.6

 
$
480.2


*
Revisions were made to estimated cash flows related to asset retirement obligations primarily due to changes in the weighted average cost to retire natural gas distribution pipe at PGL.