Benefit Plans and Other Postretirement Benefits, Postretirement Benefit Plan Benefit Obligations, Cash Flows and Benefit Costs (Details) - Other Postretirement Benefits Plan [Member] - USD ($)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in Projected Benefit Obligation [Roll Forward]
Obligation at Jan. 1		$ 41,860,000	$ 40,864,000
Service cost		875,000	775,000	$ 954,000
Interest cost		1,659,000	1,821,000	1,745,000
Medicare subsidy reimbursements		14,000	31,000
Plan participant's contributions		637,000	653,000
Actuarial loss		4,688,000	1,293,000
Benefit payments		(2,764,000)	(3,577,000)
Obligation at Dec. 31		46,969,000	41,860,000	40,864,000
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1		42,297,000	42,684,000
Actual return on plan assets		3,686,000	1,978,000
Plan participant's contributions		637,000	653,000
Employer contributions		183,000	559,000
Benefit payments		(2,764,000)	(3,577,000)
Fair value of plan assets at Dec. 31		44,039,000	42,297,000	42,684,000
Funded Status of Plans at Dec. 31 [Abstract]
Funded status	[1]	(2,930,000)	437,000
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery Rates [Abstract]
Net loss		(8,620,000)	(12,595,000)
Prior service credit		(2,229,000)	(2,630,000)
Total		(10,849,000)	(15,225,000)
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost (Credit) Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]
Current regulatory liabilities		(827,000)	(1,004,000)
Noncurrent regulatory liabilities		(10,022,000)	(14,221,000)
Total		$ (10,849,000)	$ (15,225,000)
Significant Assumptions Used to Measure Benefit Obligations [Abstract]
Discount rate for year-end valuation (as a percent)		3.62%	4.13%
Mortality table		RP 2014	RP 2014
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Pre-65		7.00%	5.50%
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Post-65		5.50%	5.50%
Ultimate health care trend assumption rate (as a percent)		4.50%
Period until ultimate trend rate is reached (in years)		5 years
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract]
One-percent increase in APBO		$ 4,559,000
One-percent decrease in APBO		(3,858,000)
One-percent increase in service and interest components		266,000
One-percent decrease in service and interest components		(225,000)
Cash Flows [Abstract]
Expected contribution to postretirement health care plans during 2018		0
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		875,000	$ 775,000	954,000
Interest cost		1,659,000	1,821,000	1,745,000
Expected return on plan assets		(2,355,000)	(2,377,000)	(2,540,000)
Amortization of prior service cost		(401,000)	(401,000)	(401,000)
Amortization of net loss		(618,000)	(583,000)	(639,000)
Net periodic pension cost		$ (840,000)	$ (765,000)	$ (881,000)
Significant Assumptions Used to Measure Costs [Abstract]
Discount rate (as a percent)		4.13%	4.65%	4.08%
Expected average long-term rate of return on assets (as a percent)		5.80%	5.80%	5.80%
Xcel Energy Inc.
Cash Flows [Abstract]
Total contributions to Xcel Energy's postretirement health care plans during the year		$ 20,000,000	$ 18,000,000	$ 18,000,000
Expected contribution to postretirement health care plans during 2018		$ 12,000,000

[1] Amounts are recognized in noncurrent liabilities and noncurrent assets on SPS’ balance sheet as of Dec. 31, 2017 and 2016