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Property, Plant and Equipment
 12 Months Ended
Dec. 31, 2017
Property, Plant and Equipment [Abstract]  
Property, Plant and Equipment [Text Block]
Property, Plant and Equipment

(a) Property, Plant and Equipment

Property, plant and equipment includes the following:
 
Weighted Average
Useful Lives
 
December 31,
 
(in years)
 
2017
 
2016
 
 
 
(in millions)
Natural Gas Distribution
28
 
$
6,735

 
$
6,219

Energy Services
27
 
102

 
83

Other property
14
 
51

 
49

Total
 
 
6,888

 
6,351

Accumulated depreciation and amortization:
 
 
 

 
 

Natural Gas Distribution
 
 
1,968

 
1,722

Energy Services
 
 
35

 
29

Other property
 
 
33

 
31

Total accumulated depreciation and amortization
 
 
2,036

 
1,782

Property, plant and equipment, net
 
 
$
4,852

 
$
4,569



(b) Depreciation and Amortization

The following table presents depreciation and amortization expense for 2017, 2016 and 2015:
 
Year Ended December 31,
 
2017
 
2016
 
2015
 
(in millions)
Depreciation expense
$
243

 
$
230

 
$
211

Amortization expense
36

 
19

 
16

Total depreciation and amortization expense
$
279

 
$
249

 
$
227



(c) AROs

A reconciliation of the changes in the ARO liability is as follows:
 
December 31,
 
2017
 
2016
 
(in millions)
Beginning balance
$
169

 
$
156

Accretion expense
7

 
8

Revisions in estimates of cash flows
67

 
5

Ending balance
$
243

 
$
169



CERC recorded AROs associated with the removal of asbestos and asbestos-containing material in its buildings. CERC also recorded AROs relating to gas pipelines abandoned in place.  The estimates of future liabilities were developed using historical information, and where available, quoted prices from outside contractors.

The increase of $67 million in the ARO from the revision in estimates in 2017 is primarily attributable to a decrease in the long-term discounts rates used in the ARO calculation for CERC. The increase of $5 million in the ARO from the revision in estimates in 2016 is primarily attributable to an increase in the labor rate associated with the abandonment of gas mains. There were no material additions or settlements during the years ended December 31, 2017 or 2016.