Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Short-term Debt [Line Items]
Amount outstanding at period end	$ 0	$ 14,000,000
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	1,000,000
Maximum amount outstanding	0	$ 34,000,000
Weighted average interest rate, computed on a daily basis (percentage)		0.41%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	700,000,000	$ 700,000,000
Amount outstanding at period end	0	14,000,000
Average amount outstanding	25,000,000	95,000,000
Maximum amount outstanding	$ 154,000,000	$ 449,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.69%	0.51%
Weighted average interest rate at period end (percentage)		0.60%