Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Short-term Debt	$ 239,000,000	$ 307,000,000
Money Pool
Short-term Debt [Line Items]
Borrowing limit	250,000,000	250,000,000
Short-term Debt	0	0
Average amount outstanding	0	25,000,000
Maximum amount outstanding	0	$ 156,000,000
Weighted average interest rate, computed on a daily basis (percentage)		1.93%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	700,000,000	$ 700,000,000
Short-term Debt	239,000,000	307,000,000
Average amount outstanding	245,000,000	55,000,000
Maximum amount outstanding	$ 414,000,000	$ 309,000,000
Weighted average interest rate, computed on a daily basis (percentage)	2.78%	2.28%
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.68%	2.95%