Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current Assets:
Cash and cash equivalents	$ 82,402	$ 83,102
Accounts receivable, less reserves	47,682	43,593
Other accounts receivables, less reserves	5,803	6,262
Unbilled revenues	23,849	28,243
Materials and supplies, at average cost	25,525	24,093
Fuel for electric generation, at average cost	4,466	6,599
Stored natural gas, at average cost	2,287	11,303
Prepaid taxes	12,436	16,215
Regulatory assets - current	9,301	9,477
Assets held for sale	0	3,080
Other current assets	7,527	8,593
Total Current Assets	221,278	240,560
Other long-term receivables	2,650	2,709
Regulatory assets	145,202	145,424
Other deferred assets and other	16,272	12,488
Property, Plant, and Equipment:
Property, plant, and equipment, net	1,497,157	1,367,585
Finance lease assets, net, Finance lease assets	16,976	2,181
Construction work in progress	24,649	139,671
Total Property, Plant, and Equipment	1,538,782	1,509,437
Investments	79,382	78,000
Total Assets	2,003,566	1,988,618
Current Liabilities:
Long-term debt due within one year	4,579	4,553
Short-term debt	6,000	13,000
Accounts payable	40,356	46,158
Accrued interest and taxes	12,189	7,384
Accrued payroll related items	9,628	13,044
Regulatory liabilities - current	21,738	13,826
Derivative liabilities	8,730	8,550
Other current liabilities	9,257	14,113
Total Current Liabilities	112,477	120,628
Other Credits:
Deferred income taxes	231,557	231,952
Investment tax credit - deferred	795	818
Regulatory liabilities	163,161	165,638
Accrued pension and other postretirement benefits	66,116	67,483
Derivative liabilities	22,880	23,980
Finance lease liabilities	17,666	1,771
Other deferred liabilities and other	67,681	66,361
Total Other Credits	569,856	558,003
Capitalization:
Common shareholders equity	828,950	816,644	$ 778,187
Long-term debt	492,283	493,343
Total Capitalization	1,321,233	1,309,987
Commitments and contingencies (see Footnote 9)
Total Liabilities and Capitalization	2,003,566	1,988,618
MGE [Member]
Current Assets:
Cash and cash equivalents	9,624	4,843
Accounts receivable, less reserves	47,682	43,593
Affiliate receivables	207	621
Other accounts receivables, less reserves	5,725	6,111
Unbilled revenues	23,849	28,243
Materials and supplies, at average cost	25,525	24,093
Fuel for electric generation, at average cost	4,466	6,599
Stored natural gas, at average cost	2,287	11,303
Prepaid taxes	12,325	15,790
Regulatory assets - current	9,301	9,477
Assets held for sale	0	3,080
Other current assets	7,486	8,541
Total Current Assets	148,477	162,294
Affiliate receivable long-term	3,177	3,177
Regulatory assets	145,202	145,424
Other deferred assets and other	17,715	14,142
Property, Plant, and Equipment:
Property, plant, and equipment, net	1,497,186	1,367,614
Finance lease assets, net, Finance lease assets	16,976	2,181
Construction work in progress	24,649	139,671
Total Property, Plant, and Equipment	1,538,811	1,509,466
Investments	415	388
Total Assets	1,853,797	1,834,891
Current Liabilities:
Long-term debt due within one year	4,579	4,553
Short-term debt	6,000	13,000
Accounts payable	40,372	46,165
Accrued interest and taxes	15,169	10,319
Accrued payroll related items	9,628	13,044
Regulatory liabilities - current	21,738	13,826
Derivative liabilities	8,730	8,550
Other current liabilities	9,054	11,614
Total Current Liabilities	115,270	121,071
Other Credits:
Deferred income taxes	203,578	204,616
Investment tax credit - deferred	795	818
Regulatory liabilities	163,161	165,638
Accrued pension and other postretirement benefits	66,116	67,483
Derivative liabilities	22,880	23,980
Finance lease liabilities	17,666	1,771
Other deferred liabilities and other	67,668	66,361
Total Other Credits	541,864	530,667
Capitalization:
Common shareholders equity	564,936	548,356
Noncontrolling interest	139,444	141,454
Total Equity	704,380	689,810	632,774
Long-term debt	492,283	493,343
Total Capitalization	1,196,663	1,183,153
Commitments and contingencies (see Footnote 9)
Total Liabilities and Capitalization	$ 1,853,797	$ 1,834,891