Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2018	Sep. 30, 2017
Regulatory Assets and Liabilities
Current regulatory assets	$ 421	$ 447
Non-current regulatory assets	8,006	8,698
Regulatory assets	8,427	9,145
Current regulatory liabilities	166	163
Non-current regulatory liabilities	19	25
Regulatory liabilities	185	188
Deferred other post-retirement benefits cost
Regulatory Assets and Liabilities
Non-current regulatory liabilities	12	23
Fuel cost adjustment tax equivalents
Regulatory Assets and Liabilities
Current regulatory liabilities	145	153
Fuel cost adjustment receivable/liability
Regulatory Assets and Liabilities
Current regulatory liabilities	0	2
Unrealized gains/losses on commodity derivatives
Regulatory Assets and Liabilities
Current regulatory liabilities	21	8
Non-current regulatory liabilities	7	2
Deferred nuclear generating units
Regulatory Assets and Liabilities
Current regulatory assets	237	237
Non-current regulatory assets	586	759
Unrealized losses on interest rate derivatives
Regulatory Assets and Liabilities
Current regulatory assets	77	93
Non-current regulatory assets	771	982
Unrealized gains/losses on commodity derivatives
Regulatory Assets and Liabilities
Current regulatory assets	17	68
Non-current regulatory assets	14	9
Fuel cost adjustment receivable/liability
Regulatory Assets and Liabilities
Current regulatory assets	5	1
Environmental agreements
Regulatory Assets and Liabilities
Current regulatory assets	4	2
Non-current regulatory assets	11	13
Environmental cleanup costs - Kingston ash spill
Regulatory Assets and Liabilities
Current regulatory assets	44	44
Non-current regulatory assets	232	263
Gallatin coal combustion residual facilities
Regulatory Assets and Liabilities
Current regulatory assets	34	0
Non-current regulatory assets	865	899
Other current regulatory assets
Regulatory Assets and Liabilities
Current regulatory assets	3	2
Deferred pension costs and other post-retirement benefits costs
Regulatory Assets and Liabilities
Non-current regulatory assets	3,817	4,009
Nuclear decommissioning costs
Regulatory Assets and Liabilities
Non-current regulatory assets	820	823
Non-nuclear decommissioning costs
Regulatory Assets and Liabilities
Non-current regulatory assets	689	703
Other non-current regulatory assets
Regulatory Assets and Liabilities
Non-current regulatory assets	$ 201	$ 238