Benefit Plans and Other Postretirement Benefits (Details) (USD $)	1 Months Ended	3 Months Ended		9 Months Ended
	Jan. 31, 2012	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Pension Benefits [Member]
Components of Net Periodic Benefit Cost [Abstract]
Service cost		$ 2,130,000	$ 1,923,000	$ 6,390,000	$ 5,768,000
Interest cost		4,900,000	5,008,000	14,700,000	15,026,000
Expected return on plan assets		(6,232,000)	(6,579,000)	(18,696,000)	(19,737,000)
Amortization of prior service cost (credit)		360,000	376,000	1,079,000	1,129,000
Amortization of net loss		3,203,000	2,262,000	9,610,000	6,785,000
Net periodic benefit cost		4,361,000	2,990,000	13,083,000	8,971,000
Cost not recognized due to the effects of regulation		(1,076,000)	(575,000)	(3,226,000)	(1,725,000)
Net benefit cost recognized for financial reporting		3,285,000	2,415,000	9,857,000	7,246,000
Pension contributions attributable to SPS	12,900,000
Postretirement Health Care Benefits [Member]
Components of Net Periodic Benefit Cost [Abstract]
Service cost		314,000	273,000	944,000	819,000
Interest cost		707,000	681,000	2,123,000	2,042,000
Expected return on plan assets		(675,000)	(752,000)	(2,026,000)	(2,255,000)
Amortization of transition obligation		386,000	417,000	1,159,000	1,252,000
Amortization of prior service cost (credit)		(37,000)	(12,000)	(111,000)	(38,000)
Amortization of net loss		314,000	213,000	942,000	641,000
Net periodic benefit cost		1,009,000	820,000	3,031,000	2,461,000
Net benefit cost recognized for financial reporting		1,009,000	820,000	3,031,000	2,461,000
Xcel Energy Inc. [Member]
Components of Net Periodic Benefit Cost [Abstract]
Total contributions to Xcel Energy's pension plans during the period	$ 190,500,000
Number of Xcel Energy's pension plans to which contributions were made	4