Fair Value of Financial Instruments (Details-2) - USD ($) $ in Thousands	Dec. 31, 2017		Dec. 31, 2016
Assets:
Available-for-sale securities	$ 7,291		$ 2,554
Liabilities:
Derivative liability, collateral offset	100		0
Recurring [Member]
Assets:
Derivative assets, net	1,005		1,527
Total Assets	1,797		2,027
Liabilities:
Derivative liabilities, net	42,963	[1]	50,808
Deferred compensation	3,065		3,039
Total Liabilities	46,028		53,847
Recurring [Member] | Exchange-traded investments
Assets:
Available-for-sale securities	792		500
Recurring [Member] | Level 1 [Member]
Assets:
Derivative assets, net	278		1,041
Total Assets	1,070		1,541
Liabilities:
Derivative liabilities, net	210	[1]	16
Deferred compensation	0		0
Total Liabilities	210		16
Recurring [Member] | Level 1 [Member] | Exchange-traded investments
Assets:
Available-for-sale securities	792		500
Recurring [Member] | Level 2 [Member]
Assets:
Derivative assets, net	0		0
Total Assets	0		0
Liabilities:
Derivative liabilities, net	0	[1]	0
Deferred compensation	3,065		3,039
Total Liabilities	3,065		3,039
Recurring [Member] | Level 2 [Member] | Exchange-traded investments
Assets:
Available-for-sale securities	0		0
Recurring [Member] | Level 3 [Member]
Assets:
Derivative assets, net	727		486
Total Assets	727		486
Liabilities:
Derivative liabilities, net	42,753	[1]	50,792
Deferred compensation	0		0
Total Liabilities	42,753		50,792
Recurring [Member] | Level 3 [Member] | Exchange-traded investments
Assets:
Available-for-sale securities	0		0
MGE [Member]
Assets:
Available-for-sale securities	409		487
Liabilities:
Derivative liability, collateral offset	100		0
MGE [Member] | Recurring [Member]
Assets:
Derivative assets, net	1,005		1,527
Total Assets	1,069		1,670
Liabilities:
Derivative liabilities, net	42,963	[1]	50,808
Deferred compensation	3,065		3,039
Total Liabilities	46,028		53,847
MGE [Member] | Recurring [Member] | Exchange-traded investments
Assets:
Available-for-sale securities	64		143
MGE [Member] | Recurring [Member] | Level 1 [Member]
Assets:
Derivative assets, net	278		1,041
Total Assets	342		1,184
Liabilities:
Derivative liabilities, net	210	[1]	16
Deferred compensation	0		0
Total Liabilities	210		16
MGE [Member] | Recurring [Member] | Level 1 [Member] | Exchange-traded investments
Assets:
Available-for-sale securities	64		143
MGE [Member] | Recurring [Member] | Level 2 [Member]
Assets:
Derivative assets, net	0		0
Total Assets	0		0
Liabilities:
Derivative liabilities, net	0	[1]	0
Deferred compensation	3,065		3,039
Total Liabilities	3,065		3,039
MGE [Member] | Recurring [Member] | Level 2 [Member] | Exchange-traded investments
Assets:
Available-for-sale securities	0		0
MGE [Member] | Recurring [Member] | Level 3 [Member]
Assets:
Derivative assets, net	727		486
Total Assets	727		486
Liabilities:
Derivative liabilities, net	42,753	[1]	50,792
Deferred compensation	0		0
Total Liabilities	42,753		50,792
MGE [Member] | Recurring [Member] | Level 3 [Member] | Exchange-traded investments
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.