Long-term and Short-term Debt Long-term and Short-term Debt (Narrative - Line of Credit) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2037	Dec. 01, 2037	Dec. 31, 2024	Dec. 31, 2023	Jun. 30, 2023	Dec. 31, 2022	Jun. 30, 2022
Committed Credit Facility Six [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	$ 413,000,000
Line of Credit Facility, Maximum Borrowing Capacity	2,896,000,000
Line of Credit Facility, Current Borrowing Capacity	2,800,000,000
General Credit Facility Three [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	2,537,000,000
Line of Credit Facility, Maximum Borrowing Capacity	3,000,000,000
Committed Credit Facility Three [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	4,000,000
Line of Credit Facility, Maximum Borrowing Capacity	400,000,000
Committed Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	417,000,000
Line of Credit Facility, Maximum Borrowing Capacity	3,296,000,000
Committed Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	3,300,000,000
Line of Credit Facility, Commitment Fee Amount	12,000,000	$ 12,000,000	$ 15,000,000
General Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Commitment Fee Amount	$ 14,000,000	$ 12,000,000	$ 10,000,000
Forecast [Member] | Committed Credit Facility Six [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity				$ 0	$ 2,000,000,000.0
Forecast [Member] | Committed Credit Facility Three [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity						$ 0	$ 260,000,000	$ 310,000,000	$ 360,000,000	$ 380,000,000