Commitments and Contingent Liabilities, Leases (Details) (USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2009
Leases [Abstract]
Number of leases qualifying as capital leases	3
Amortization expense under capital lease assets	$ 3,200,000		$ 5,300,000	$ 3,500,000
Property Held Under Capital Leases, Net [Abstract]
Property held under capital lease	221,200,000		216,800,000
Accumulated depreciation	(29,800,000)		(26,600,000)
Total property held under capital leases, net	191,400,000		190,200,000
Operating leases, future minimum payments due [Abstract]
2012	185,600,000
2013	198,300,000
2014	204,900,000
2015	205,200,000
2016	192,100,000
Thereafter	1,998,300,000
Capital leases, future minimum payments due [Abstract]
2012	18,200,000
2013	18,000,000
2014	18,000,000
2015	17,900,000
2016	17,200,000
Thereafter	306,200,000
Total minimum obligation	395,500,000
Interest component of obligation	(280,500,000)
Present value of minimum obligation	115,000,000	[1]
WYCO Totem Gas Storage Facilities [Member]
Leases [Abstract]
Ownership interest in joint venture (in hundredths)	50.00%
Capital lease obligations	152,700,000		149,900,000
Percentage of the capital lease obligation related to WYCO eliminated (in hundredths)	50.00%
Storage, Leaseholds and Rights [Member]
Property Held Under Capital Leases, Net [Abstract]
Property held under capital lease	200,500,000		196,100,000
Gas Pipeline [Member]
Property Held Under Capital Leases, Net [Abstract]
Property held under capital lease	20,700,000		20,700,000
Other Operating Leases [Member]
Leases [Abstract]
Total expenses under operating lease obligations	204,800,000		197,400,000	209,500,000
Payments for capacity for purchase power agreements under operating lease obligations	160,500,000		163,700,000	171,300,000
Operating leases, future minimum payments due [Abstract]
2012	26,600,000
2013	24,800,000
2014	24,300,000
2015	23,200,000
2016	18,200,000
Thereafter	89,600,000
Purchase Power Agreement Operating Leases [Member]
Operating leases, future minimum payments due [Abstract]
2012	159,000,000	[2],[3]
2013	173,500,000	[2],[3]
2014	180,600,000	[2],[3]
2015	182,000,000	[2],[3]
2016	173,900,000	[2],[3]
Thereafter	$ 1,908,700,000	[2],[3]

[1] Future commitments exclude certain amounts related to Xcel Energy's 50 percent ownership interest in WYCO.

[2] Purchased power agreement operating leases contractually expire through 2033.

[3] Amounts do not include purchased power agreements accounted for as other commitments, which are recorded to O&M as executed.