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Property, Plant and Equipment (Tables)
 12 Months Ended
Dec. 31, 2012
Property, Plant and Equipment [Abstract]  
Property, Plant, and Equipment [Table Text Block]
Property, Plant and Equipment
 
 
 
As of December 31
2012
 
2011
Millions
 
 
 
Regulated Utility

$3,232.9

 

$2,794.8

Construction Work in Progress
151.8

 
155.0

Accumulated Depreciation
(1,102.8
)
 
(1,024.6
)
Regulated Utility Plant - Net
2,281.9

 
1,925.2

Non-Rate Base Energy Operations
118.0

 
106.4

Construction Work in Progress
4.2

 
2.3

Accumulated Depreciation
(56.7
)
 
(51.4
)
Non-Rate Base Energy Operations Plant - Net
65.5

 
57.3

Other Plant - Net
0.2

 
0.2

Property, Plant and Equipment - Net

$2,347.6

 

$1,982.7
Estimated Useful Lives of Property, Plant and Equipment [Table Text Block]
Estimated Useful Lives of Property, Plant and Equipment
 
 
 
 
 
Regulated Utility –
Generation
3 to 35 years
Non-Rate Base Operations
3 to 61 years
 
Transmission
42 to 61 years
Other Plant
5 to 25 years
 
Distribution
14 to 65 years
 
 
Asset Retirement Obligation [Table Text Block]
Asset Retirement Obligation
 
 
Millions
 
 
Obligation as of December 31, 2010
 

$50.3

Accretion Expense
 
6.4

Additional Liabilities Incurred in 2011
 
0.3

Obligation as of December 31, 2011
 
57.0

Accretion Expense
 
3.8

Additional Liabilities Incurred in 2012
 
17.1

Obligation as of December 31, 2012
 

$77.9