Exit Activities and Discontinued Operations - Assets and Liabilities of Discontinued Operation (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Current
Total current assets	$ 494	$ 429
Non‑current
Total non‑current assets	2,606	2,680
Current
Total current liabilities	1,233	953
Non‑current
Total non‑current liabilities	1,523	1,894
Discontinued Operations, Held-for-sale
Current
Cash and cash equivalents	104	136
Discontinued Operations, Held-for-sale | EnerBank
Current
Cash and cash equivalents	104	136
Accounts receivable and other current assets	143	18
Notes receivable, allowance, current	28	32
Notes receivable, less allowance of $28 in 2021 and $32 in 2020	247	275
Total current assets	494	429
Non‑current
Plant, property, and equipment, net	29	22
Notes receivable, allowance, noncurrent	84	91
Notes receivable, less allowance of $84 in 2021 and $91 in 2020	2,532	2,612
Other non‑current assets	45	46
Total non‑current assets	2,606	2,680
Total assets	3,100	3,109
Current
Current portion of long-term debt	1,199	915
Accounts payable and other current liabilities	34	38
Total current liabilities	1,233	953
Non‑current
Long-term debt	1,522	1,890
Other non‑current liabilities	1	4
Total non‑current liabilities	1,523	1,894
Total liabilities	$ 2,756	$ 2,847