Revenue Local Power Company Contracts (Details) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020 USD ($) percent	Mar. 31, 2019 USD ($)	Mar. 31, 2020 USD ($) percent	Mar. 31, 2019 USD ($)
Long-term Contract for Purchase of Electric Power [Line Items]
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block]	$ 2,521	$ 2,750	$ 5,099	$ 5,475
Total number of customers that represent the percent of sales \| percent	1		1
Total long-duration contracts revenue recognition	2,301		4,658
5-year Termination Notice [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Number of customers that represent the percent of sales \| percent	0		0
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block]	$ 549		$ 1,122
10-year Termination Notice [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Number of customers that represent the percent of sales \| percent	0		0
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block]	$ 15		$ 30
20-year Termination Notice [Member]
Long-term Contract for Purchase of Electric Power [Line Items]
Number of customers that represent the percent of sales \| percent	1		1
Long-Duration Contracts Revenue Recognition, Policy [Policy Text Block]	$ 1,737		$ 3,506