New Accounting Standards (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption		$ 0
Accumulated Other Comprehensive income (loss)	$ 0	377
MGE [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption		0
Accumulated Other Comprehensive income (loss)	0	(28)
Accounting Standards Update 2016-01 - Financial Instruments - Overall [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	377
Accumulated Other Comprehensive income (loss)	(377)
Accounting Standards Update 2016-01 - Financial Instruments - Overall [Member] | MGE [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	(28)
Accumulated Other Comprehensive income (loss)	28
Accounting Standards Update 2017-07 - Compensation - Retirement Benefits [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption		$ 4,100	$ 4,300
Accounting Standards Update 2016-02 - Leases [Member] | Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New accounting pronouncement, effect of adoption, in assets and liabilities (leases)	15,000
Accounting Standards Update 2016-02 - Leases [Member] | Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New accounting pronouncement, effect of adoption, in assets and liabilities (leases)	$ 17,000