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Asset Retirement Obligations Asset Retirement Obligations (Tables)
 12 Months Ended
Sep. 30, 2018
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligation Activity
Asset Retirement Obligation Activity
 
Nuclear
 
Non-Nuclear
 
Total
Balance at September 30, 2016
$
2,492

 
$
1,560

 
$
4,052

 
Settlements
—

 
(123
)
 
(123
)
 
Change in estimate
250

 
161

 
411

 
Additional obligations
—

 
1

 
1

 
Reclassification of Gallatin projects(2)
—

 
(188
)
 
(188
)
 
Accretion (recorded to regulatory asset)
117

 
34

 
151

 
Balance at September 30, 2017
2,859

 
1,445

 
4,304

(1) 
Settlements
—

 
(106
)
 
(106
)
 
Change in estimate
—

 
430

 
430

 
Additional obligations
—

 
1

 
1

 
Accretion (recorded to regulatory asset)
130

 
35

 
165

 
Asset Disposition
—


(15
)
 
(15
)
 
Balance at September 30, 2018
$
2,989

 
$
1,790

 
$
4,779

(1) 

Notes
(1) The current portions of the ARO liability in the amounts of $115 million and $128 million as of September 30, 2018 and 2017, respectively, are included in Accounts payable and accrued liabilities.
(2) See Note 8 for additional information