Fair Value of Financial Instruments (Details-2) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Liabilities:
Derivative liability, collateral offset	$ 0	$ 100
Recurring [Member]
Assets:
Derivatives	1,927	1,005
Total Assets	2,932	1,797
Liabilities:
Derivatives	36,557	42,963	[1]
Deferred compensation	2,985	3,065
Total liabilities	39,542	46,028
Recurring [Member] | Exchange Traded [Member]
Assets:
Available-for-sale securities	1,005	792
Recurring [Member] | Level 1 [Member]
Assets:
Derivatives	824	278
Total Assets	1,829	1,070
Liabilities:
Derivatives	140	210	[1]
Deferred compensation	0	0
Total liabilities	140	210
Recurring [Member] | Level 1 [Member] | Exchange Traded [Member]
Assets:
Available-for-sale securities	1,005	792
Recurring [Member] | Level 2 [Member]
Assets:
Derivatives	0	0
Total Assets	0	0
Liabilities:
Derivatives	0	0	[1]
Deferred compensation	2,985	3,065
Total liabilities	2,985	3,065
Recurring [Member] | Level 2 [Member] | Exchange Traded [Member]
Assets:
Available-for-sale securities	0	0
Recurring [Member] | Level 3 [Member]
Assets:
Derivatives	1,103	727
Total Assets	1,103	727
Liabilities:
Derivatives	36,417	42,753	[1]
Deferred compensation	0	0
Total liabilities	36,417	42,753
Recurring [Member] | Level 3 [Member] | Exchange Traded [Member]
Assets:
Available-for-sale securities	0	0
MGE [Member]
Liabilities:
Derivative liability, collateral offset	0	100
MGE [Member] | Recurring [Member]
Assets:
Derivatives	1,927	1,005
Total Assets	1,997	1,069
Liabilities:
Derivatives	36,557	42,963	[1]
Deferred compensation	2,985	3,065
Total liabilities	39,542	46,028
MGE [Member] | Recurring [Member] | Exchange Traded [Member]
Assets:
Available-for-sale securities	70	64
MGE [Member] | Recurring [Member] | Level 1 [Member]
Assets:
Derivatives	824	278
Total Assets	894	342
Liabilities:
Derivatives	140	210	[1]
Deferred compensation	0	0
Total liabilities	140	210
MGE [Member] | Recurring [Member] | Level 1 [Member] | Exchange Traded [Member]
Assets:
Available-for-sale securities	70	64
MGE [Member] | Recurring [Member] | Level 2 [Member]
Assets:
Derivatives	0	0
Total Assets	0	0
Liabilities:
Derivatives	0	0	[1]
Deferred compensation	2,985	3,065
Total liabilities	2,985	3,065
MGE [Member] | Recurring [Member] | Level 2 [Member] | Exchange Traded [Member]
Assets:
Available-for-sale securities	0	0
MGE [Member] | Recurring [Member] | Level 3 [Member]
Assets:
Derivatives	1,103	727
Total Assets	1,103	727
Liabilities:
Derivatives	36,417	42,753	[1]
Deferred compensation	0	0
Total liabilities	36,417	42,753
MGE [Member] | Recurring [Member] | Level 3 [Member] | Exchange Traded [Member]
Assets:
Available-for-sale securities	$ 0	$ 0

[1] These amounts are shown gross and exclude $0.1 million of collateral that was posted against derivative positions with counterparties as of December 31, 2017.