Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Short-term debt	$ 307,000,000	$ 307,000,000	$ 0
Money Pool
Short-term Debt [Line Items]
Borrowing limit	250,000,000	250,000,000	250,000,000	$ 250,000,000
Short-term debt	0	0	0	0
Average amount outstanding	26,000,000	25,000,000	0	21,000,000
Maximum amount outstanding	$ 96,000,000	$ 156,000,000	$ 20,000,000	$ 141,000,000
Short-term Debt, Weighted Average Interest Rate, over Time	2.27%	1.93%	0.92%	0.73%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 700,000,000	$ 700,000,000	$ 700,000,000	$ 700,000,000
Short-term debt	307,000,000	307,000,000	0	129,000,000
Average amount outstanding	87,000,000	55,000,000	54,000,000	24,000,000
Maximum amount outstanding	$ 309,000,000	$ 309,000,000	$ 268,000,000	$ 154,000,000
Short-term Debt, Weighted Average Interest Rate, over Time	2.64%	2.28%	1.08%	0.70%
Weighted average interest rate at period end (percentage)	2.95%	2.95%		0.95%