Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Activities:
Net Income	$ 86,874,000	$ 84,219,000	$ 97,606,000
Items not affecting cash:
Depreciation and amortization	71,562,000	56,412,000	53,077,000
Deferred income taxes	7,212,000	3,749,000	(4,082,000)
Provision for doubtful receivables	1,615,000	1,366,000	1,016,000
Employee benefit plan (credit) cost	(3,813,000)	(2,002,000)	831,000
Equity earnings in ATC	(9,889,000)	(8,821,000)	(10,125,000)
Loss (gain) on sale of property	406,000	0	(1,547,000)
Other items	(243,000)	(814,000)	641,000
Changes in working capital items:
Trade and other receivables (change)	(1,285,000)	(2,235,000)	(4,502,000)
Inventories (change)	(3,420,000)	2,724,000	(2,568,000)
Unbilled revenues (change)	2,345,000	3,157,000	(1,554,000)
Prepaid taxes (change)	(677,000)	10,320,000	101,000
Other current assets (change)	29,000	251,000	(2,786,000)
Accounts payable (change)	(7,432,000)	(4,855,000)	(5,290,000)
Other current liabilities (change)	(10,264,000)	11,957,000	3,948,000
Dividends from investment	7,347,000	6,958,000	8,803,000
Cash contributions to pension and other postretirement plans	(5,714,000)	(5,584,000)	(11,304,000)
Other noncurrent items, net	(4,178,000)	(3,762,000)	9,108,000
Cash Provided by Operating Activities	130,475,000	153,040,000	131,373,000
Investing Activities:
Capital expenditures	(164,036,000)	(212,197,000)	(108,131,000)
Capital contributions to investments	(7,813,000)	(5,926,000)	(11,081,000)
Proceeds from sale of property	0	0	2,819,000
Other	(508,000)	(205,000)	118,000
Cash Used for Investing Activities	(172,357,000)	(218,328,000)	(116,275,000)
Financing Activities:
Cash dividends paid on common stock	(47,842,000)	(45,762,000)	(43,682,000)
Repayment of long-term debt	(4,553,000)	(24,453,000)	(34,358,000)
Issuance of long-term debt	50,000,000	100,000,000	70,000,000
(Repayments of) Proceeds from short-term debt	(13,000,000)	9,000,000	4,000,000
Other	(1,838,000)	(662,000)	(597,000)
Cash Provided by (Used for) Financing Activities	(17,233,000)	38,123,000	(4,637,000)
Change in cash, cash equivalents, and restricted cash:	(59,115,000)	(27,165,000)	10,461,000
Cash, cash equivalents, and restricted cash at beginning of period	84,929,000	112,094,000	101,633,000
Cash, cash equivalents, and restricted cash at end of period	25,814,000	84,929,000	112,094,000
Supplemental disclosures of cash flow information:
Interest paid	23,171,000	20,018,000	19,176,000
Income taxes paid	14,617,000	12,597,000	27,149,000
Income taxes received	(255,000)	(59,000)	0
Significant noncash investing activities:
Accrued capital expenditures	26,697,000	11,129,000	16,602,000
MGE [Member]
Operating Activities:
Net Income	80,779,000	79,036,000	93,072,000
Items not affecting cash:
Depreciation and amortization	71,562,000	56,412,000	53,077,000
Deferred income taxes	5,286,000	(69,000)	(7,760,000)
Provision for doubtful receivables	1,615,000	1,366,000	1,016,000
Employee benefit plan (credit) cost	(3,813,000)	(2,002,000)	831,000
Loss (gain) on sale of property	406,000	0	(1,547,000)
Other items	850,000	(107,000)	1,408,000
Changes in working capital items:
Trade and other receivables (change)	(1,285,000)	(2,160,000)	(4,113,000)
Inventories (change)	(3,420,000)	2,724,000	(2,568,000)
Unbilled revenues (change)	2,345,000	3,157,000	(1,554,000)
Prepaid taxes (change)	327,000	9,283,000	725,000
Other current assets (change)	147,000	255,000	(2,788,000)
Accounts payable (change)	(7,754,000)	(4,817,000)	(5,312,000)
Accrued interest and taxes (change)	(1,565,000)	4,761,000	118,000
Other current liabilities (change)	(9,618,000)	9,685,000	1,941,000
Cash contributions to pension and other postretirement plans	(5,714,000)	(5,584,000)	(11,304,000)
Other noncurrent items, net	(4,278,000)	(3,946,000)	8,878,000
Cash Provided by Operating Activities	125,870,000	147,994,000	124,120,000
Investing Activities:
Capital expenditures	(164,036,000)	(212,197,000)	(108,131,000)
Proceeds from sale of property	0	0	1,721,000
Other	(782,000)	(1,105,000)	(162,000)
Cash Used for Investing Activities	(164,818,000)	(213,302,000)	(106,572,000)
Financing Activities:
Cash dividends paid to parent by MGE	0	0	(45,000,000)
Distributions to parent from noncontrolling interest	(23,500,000)	(22,000,000)	(18,000,000)
Capital contributions from parent	30,500,000	0	0
Repayment of long-term debt	(4,553,000)	(24,453,000)	(34,358,000)
Issuance of long-term debt	50,000,000	100,000,000	70,000,000
(Repayments of) Proceeds from short-term debt	(13,000,000)	9,000,000	4,000,000
Other	(1,640,000)	(662,000)	(539,000)
Cash Provided by (Used for) Financing Activities	37,807,000	61,885,000	(23,897,000)
Change in cash, cash equivalents, and restricted cash:	(1,141,000)	(3,423,000)	(6,349,000)
Cash, cash equivalents, and restricted cash at beginning of period	6,670,000	10,093,000	16,442,000
Cash, cash equivalents, and restricted cash at end of period	5,529,000	6,670,000	10,093,000
Supplemental disclosures of cash flow information:
Interest paid	23,171,000	20,018,000	19,176,000
Income taxes paid	95,000	0	1,000
Income taxes received	(249,000)	(59,000)	0
Significant noncash investing activities:
Accrued capital expenditures	$ 26,697,000	$ 11,129,000	$ 16,602,000