Commitments and Contingencies, Leases (Details) $ in Millions		12 Months Ended
		Dec. 31, 2015 USD ($) Lease	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Capital Leases [Abstract]
Number of leases qualifying as capital leases | Lease		3
Amortization expense under capital lease assets		$ 8.2	$ 7.2	$ 6.3
Property Held Under Capital Leases, Net [Abstract]
Property held under capital lease		221.2	221.2
Accumulated depreciation		(57.2)	(49.0)
Total property held under capital leases, net		$ 164.0	172.2
Operating Leased Assets [Line Items]
Operating Lease Purchase Power Agreement Expiration (year)		2039
Operating Leases, Future Minimum Payments Due [Abstract]
2016		$ 241.6
2017		230.7
2018		231.5
2020		262.1
2019		253.7
Thereafter		2,310.2
Capital Leases, Future Minimum Payments Due [Abstract]
2016		17.1
2017		15.1
2018		14.7
2019		14.5
2020		14.3
Thereafter		258.8
Total minimum obligation		334.5
Interest component of obligation		(236.9)
Present value of minimum obligation	[1]	$ 97.6
WYCO Totem Gas Storage Facilities
Capital Leases [Abstract]
Ownership interest in joint venture (in hundredths)		50.00%
Capital Lease Obligations		$ 132.9	138.9
Percentage of the capital lease obligation related to WYCO eliminated (in hundredths)		50.00%
Gas Storage Facilities
Property Held Under Capital Leases, Net [Abstract]
Property held under capital lease		$ 200.5	200.5
Gas Pipeline
Property Held Under Capital Leases, Net [Abstract]
Property held under capital lease		20.7	20.7
Office Space and Other Equipment
Operating Leases [Abstract]
Total expenses under operating lease obligations		265.3	271.9	242.1
Operating Leases, Future Minimum Payments Due [Abstract]
2016		24.6
2017		22.0
2018		21.5
2020		20.8
2019		25.9
Thereafter		175.0
Purchased Power Agreements
Operating Leases [Abstract]
Payments for capacity for PPAs under operating lease obligations		223.6	$ 228.2	$ 197.7
Operating Leases, Future Minimum Payments Due [Abstract]
2016	[2],[3]	217.0
2017	[2],[3]	208.7
2018	[2],[3]	210.0
2020	[2],[3]	241.3
2019	[2],[3]	227.8
Thereafter	[2],[3]	$ 2,135.2

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

[2] Amounts do not include PPAs accounted for as executory contracts.

[3] PPA operating leases contractually expire through 2039.