Commitments, Guarantees and Contingencies - Environmental Matters (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) MW	Dec. 31, 2017 USD ($)
NOV Consent Decree [Member]
Environmental Matters [Line Items]
Consent Decree Wind Energy Generation Addition (MW) | MW	200
Coal Combustion Residuals [Member] | Minnesota Power [Member]
Environmental Matters [Line Items]
Expected Period for Costs of Compliance	15 years
Coal Combustion Residuals [Member] | Minimum [Member] | Minnesota Power [Member]
Environmental Matters [Line Items]
Estimated Costs of Compliance	$ 65
Coal Combustion Residuals [Member] | Maximum [Member] | Minnesota Power [Member]
Environmental Matters [Line Items]
Estimated Costs of Compliance	120
Manufactured Gas Plant [Member] | SWL&P [Member] | Superior, WI [Member]
Environmental Matters [Line Items]
Estimated Costs of Compliance (Accrued)	$ 7	$ 8
Expected Period to Incur Remediation Costs	4 years