Commitments and Contingencies, Environmental Contingencies - Unrecorded Unconditional Purchase Obligation (Details) $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($)
Regional Haze Rules
Environmental Requirements [Abstract]
Length of Time States Are Required to Revise Plans (in Years)	10 years
Capital Commitments | NSP Minnesota | Federal Clean Water Act Effluent Limitations Guidelines [Member]
Environmental Requirements [Abstract]
Liability for Estimated Cost to Comply with Regulation	$ 10
Minimum | Capital Commitments | Federal Clean Water Act Effluent Limitations Guidelines [Member]
Environmental Requirements [Abstract]
Liability for Estimated Cost to Comply with Regulation	21
Maximum | Capital Commitments | Federal Clean Water Act Effluent Limitations Guidelines [Member]
Environmental Requirements [Abstract]
Liability for Estimated Cost to Comply with Regulation	32
Harrington Units 1 and 2 [Member] | Capital Commitments | SPS | Regional Haze Rules
Environmental Requirements [Abstract]
Liability for Estimated Cost to Comply with Regulation	400
Tolk Units 1 and 2 [Member] | Capital Commitments | SPS | Regional Haze Rules
Environmental Requirements [Abstract]
Liability for Estimated Cost to Comply with Regulation	$ 600