Fair Value of Financial Instruments Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash Surrender Value, Fair Value Disclosure	$ 35		$ 35		$ 33
Long-term Debt	3,285		3,285		3,046
Unamortized Debt Issuance Expense	14		14		13
Long-term Debt, Fair Value	3,854		3,854		$ 3,808
Net gain or loss recognized in the statement of income offset by regulatory accounting	(114)	$ 63	(265)	$ (22)
net realized loss	$ 2		$ 4
net realized gains		$ 5		$ 8