Adoption of Accounting Principles and Recently Issued Accounting Pronouncements (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Restricted cash	$ 2,100	$ 3,700
Accumulated Other Comprehensive income (loss)	377	202
Cumulative Effect of New Accounting Principle in Period of Adoption				$ (142)
MGE [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated Other Comprehensive income (loss)	(28)	19
Cumulative Effect of New Accounting Principle in Period of Adoption				$ (142)
Accounting Standards Update 2016-18 - Restricted Cash [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Restricted cash	4,600	5,100
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		$ 6,600	$ 5,000
AAccounting Standards Update 2016-01 - Financial Instruments - Overall [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated Other Comprehensive income (loss)	377
AAccounting Standards Update 2016-01 - Financial Instruments - Overall [Member] | MGE [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated Other Comprehensive income (loss)	$ (28)