Schedule I - Condensed Parent Company Finanical Statements (Details-Balance Sheet) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current Assets:
Cash and cash equivalents	$ 107,952	$ 95,959	$ 81,384	$ 65,755
Accounts receivable, net:
Other current assets	12,507	12,293
Total Current Assets	290,631	275,515
Other deferred assets and other	731	1,088
Investments:
Other investments	55	120
Total investments	67,772	76,290
Total Assets	1,855,182	1,801,060	1,726,403
Current Liabilities:
Other current liabilities	18,758	19,456
Total Current Liabilities	126,514	103,505
Other Credits:
Deferred income taxes	225,130	383,813
Total Other Credits	552,320	590,676
Shareholders' Equity:
Retained earnings	426,874	372,950
Accumulated other comprehensive income, net of tax	377	202
Total Common Shareholders' Equity	778,187	724,088	690,458	659,401
Commitments and contingencies (see Footnote 3)
Total Liabilities and Capitalization	1,855,182	1,801,060
MGE Energy [Member]
Current Assets:
Cash and cash equivalents	98,540	82,225	$ 51,781	$ 58,429
Accounts receivable, net:
Accounts receivable from affiliates	0	83
Other current assets	1,601	1,221
Total Current Assets	100,141	83,529
Other deferred assets and other	171	201
Investments:
Investments in affiliates	702,134	684,968
Other investments	5,640	1,161
Total investments	707,774	686,129
Total Assets	808,086	769,859
Current Liabilities:
Accounts payable to affiliates	665	530
Accrued taxes	17	218
Other current liabilities	2,030	115
Total Current Liabilities	2,712	863
Other Credits:
Deferred income taxes	23,480	40,672
Accounts payable to affiliates	3,707	4,236
Total Other Credits	27,187	44,908
Shareholders' Equity:
Common shareholders' equity	350,936	350,936
Retained earnings	426,874	372,950
Accumulated other comprehensive income, net of tax	377	202
Total Common Shareholders' Equity	778,187	724,088
Commitments and contingencies (see Footnote 3)
Total Liabilities and Capitalization	$ 808,086	$ 769,859