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	$ 73,000,000	$ 223,000,000
Money Pool
Short-term Debt [Line Items]
Borrowing limit	250,000,000	250,000,000
Amount outstanding at period end	93,000,000	0
Average amount outstanding	39,000,000	5,000,000
Maximum amount outstanding	$ 225,000,000	$ 69,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.71%	0.53%
Weighted average interest rate at period end (percentage)	0.71%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 500,000,000	$ 500,000,000
Amount outstanding at period end	73,000,000	223,000,000
Average amount outstanding	224,000,000	96,000,000
Maximum amount outstanding	$ 353,000,000	$ 327,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.66%	0.43%
Weighted average interest rate at period end (percentage)	0.55%	0.72%