Commitments and Contingencies (Details)	6 Months Ended
	Jun. 30, 2018 USD ($)
CERC Corp [Member] | Minnesota Service Territory [Member]
Legal, Environmental and Other Matters
Liability recorded for remediation of Minnesota sites	$ 7,000,000
CERC Corp [Member] | Minnesota Service Territory [Member] | Minimum [Member]
Legal, Environmental and Other Matters
Estimated remediation costs for the Minnesota sites	$ 4,000,000
Years to resolve contingency	30 years
CERC Corp [Member] | Minnesota Service Territory [Member] | Maximum [Member]
Legal, Environmental and Other Matters
Estimated remediation costs for the Minnesota sites	$ 30,000,000
Years to resolve contingency	50 years
Natural Gas Supply Commitments [Member]
Natural Gas Supply Commitments
Remaining six months of 2018	$ 185,000,000
2019	264,000,000
2020	168,000,000
2021	82,000,000
2022	51,000,000
2023 and beyond	123,000,000
Minnehaha Academy Gas Explosion [Member]
Legal, Environmental and Other Matters
Contested amount of fines imposed	$ 200,000