Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Amount outstanding at period end	$ 31,000,000	$ 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	1,000,000	21,000,000
Maximum amount outstanding	$ 20,000,000	$ 141,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.92%	0.73%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 700,000,000	$ 700,000,000
Amount outstanding at period end	31,000,000	129,000,000
Average amount outstanding	109,000,000	24,000,000
Maximum amount outstanding	$ 268,000,000	$ 154,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.97%	0.70%
Weighted average interest rate at period end (percentage)	1.10%	0.95%