Commitments and Contingencies (Details) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($)
Minnesota Service Territory [Member]
LegalEnvironmentalAndOtherRegulatoryMattersLegalMattersGasMarketManipulationCasesAbstract
Liability recorded for remediation of Minnesota sites	$ 7
Minnesota Service Territory [Member] | Minimum [Member]
LegalEnvironmentalAndOtherRegulatoryMattersLegalMattersGasMarketManipulationCasesAbstract
Estimated remediation costs for the Minnesota sites	$ 4
Years to resolve contingency	30 years
Minnesota Service Territory [Member] | Maximum [Member]
LegalEnvironmentalAndOtherRegulatoryMattersLegalMattersGasMarketManipulationCasesAbstract
Estimated remediation costs for the Minnesota sites	$ 30
Years to resolve contingency	50 years
Natural Gas Supply Commitments [Member]
Recorded Unconditional Purchase Obligation Payment Schedule [Abstract]
Remaining three months of 2017	$ 169
2018	507
2019	348
2020	166
2021	76
2022 and beyond	$ 113