Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Amount outstanding at period end	$ 0	$ 129,000,000
Money Pool
Short-term Debt [Line Items]
Borrowing limit	250,000,000	250,000,000
Amount outstanding at period end	0	0
Average amount outstanding	0	21,000,000
Maximum amount outstanding	0	$ 141,000,000
Weighted average interest rate, computed on a daily basis (percentage)		0.73%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	700,000,000	$ 700,000,000
Amount outstanding at period end	0	129,000,000
Average amount outstanding	0	24,000,000
Maximum amount outstanding	$ 0	$ 154,000,000
Weighted average interest rate, computed on a daily basis (percentage)		0.70%
Weighted average interest rate at period end (percentage)		0.95%