Fair Value of Financial Instruments (Details-1) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Fair Value Measurement [Domain]
Liabilities:
Unamortized discount and debt issuance costs, net		$ 4,600	$ 4,300
Carrying Amount [Member]
Assets:
Cash and cash equivalents		133,855	107,952
Liabilities:
Short-term debt - commercial paper		0	4,000
Long-term debt	[1]	503,553	426,883
Fair Value [Member]
Assets:
Cash and cash equivalents		133,855	107,952
Liabilities:
Short-term debt - commercial paper		0	4,000
Long-term debt	[1]	523,075	475,282
MGE [Member] | Fair Value Measurement [Domain]
Liabilities:
Unamortized discount and debt issuance costs, net		4,600	4,300
MGE [Member] | Carrying Amount [Member]
Assets:
Cash and cash equivalents		47,264	5,951
Liabilities:
Short-term debt - commercial paper		0	4,000
Long-term debt	[1]	503,553	426,883
MGE [Member] | Fair Value [Member]
Assets:
Cash and cash equivalents		47,264	5,951
Liabilities:
Short-term debt - commercial paper		0	4,000
Long-term debt	[1]	$ 523,075	$ 475,282

[1] Includes long-term debt due within one year. Excludes debt issuance costs and unamortized discount of $4.6 million and $4.3 million at September 30, 2018 and December 31, 2017, respectively.