Commitments and Contingencies, Leases (Details) $ in Millions		12 Months Ended
		Dec. 31, 2016 USD ($) Lease	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Capital Leases [Abstract]
Number of leases qualifying as capital leases | Lease		3
Amortization expense under capital lease assets		$ 8.1	$ 8.2	$ 7.2
Property Held Under Capital Leases, Net [Abstract]
Property held under capital lease		221.2	221.2
Accumulated depreciation		(65.3)	(57.2)
Total property held under capital leases, net		155.9	164.0
Capital Leases, Future Minimum Payments Due [Abstract]
2017		15.1
2018		14.7
2019		14.5
2020		14.3
2021		13.7
Thereafter		245.0
Total minimum obligation		317.3
Interest component of obligation		(224.9)
Present value of minimum obligation	[1]	$ 92.4
Operating Leased Assets [Line Items]
Operating Lease Purchase Power Agreement Expiration Date		2039
Operating Leases, Future Minimum Payments Due [Abstract]
2017		$ 237.5
2018		238.0
2019		260.3
2020		270.1
2021		268.6
Thereafter		$ 2,089.5
WYCO Totem Gas Storage Facilities
Capital Leases [Abstract]
Ownership interest in joint venture (in hundredths)		50.00%
Capital Lease Obligations		$ 127.0	132.9
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		255.3	265.3	271.9
Operating Leases, Future Minimum Payments Due [Abstract]
2017		25.2
2018		25.2
2019		29.7
2020		23.5
2021		24.4
Thereafter		170.1
Purchased Power Agreements
Operating Leases [Abstract]
Payments for capacity for PPAs under operating lease obligations		216.4	$ 223.6	$ 228.2
Operating Leases, Future Minimum Payments Due [Abstract]
2017	[2],[3]	212.3
2018	[2],[3]	212.8
2019	[2],[3]	230.6
2020	[2],[3]	246.6
2021	[2],[3]	244.2
Thereafter	[2],[3]	$ 1,919.4

[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.