Short-Term Debt (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Short-term Debt	$ 150,000,000.0	$ 30,000,000.0	$ 150,000,000.0
Money Pool [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	250,000,000	250,000,000	250,000,000	$ 250,000,000
Short-term Debt	0	0	0	85,000,000
Short-term Debt, Average Outstanding Amount	34,000,000	32,000,000	17,000,000	25,000,000
Short-term Debt, Maximum Amount Outstanding During Period	$ 119,000,000	$ 250,000,000	$ 143,000,000	$ 142,000,000
Short-term Debt, Weighted Average Interest Rate, over Time	1.67%	2.05%	1.96%	1.14%
Short-term Debt, Weighted Average Interest Rate, at Point in Time				1.18%
Commercial Paper [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 500,000,000	$ 500,000,000	$ 500,000,000	$ 500,000,000
Short-term Debt	150,000,000	30,000,000	150,000,000	20,000,000
Short-term Debt, Average Outstanding Amount	2,000,000	71,000,000	38,000,000	62,000,000
Short-term Debt, Maximum Amount Outstanding During Period	$ 30,000,000	$ 317,000,000	$ 198,000,000	$ 237,000,000
Short-term Debt, Weighted Average Interest Rate, over Time	2.05%	2.59%	2.08%	1.10%
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.97%	2.05%	2.97%	1.93%