Variable Interest Entities (Details) $ in Thousands		Dec. 31, 2018 USD ($) MW	Dec. 31, 2017 USD ($) MW
Significant Balance Sheet Accounts Related to Interests in Variable Interest Entities
Property, plant, and equipment, net		$ 1,369,766	$ 1,283,313
Construction work in progress		139,671	58,044
Accrued interest and taxes		7,384	5,602
Deferred income taxes		231,952	225,130
Unamortized discount and debt issuance costs, net		$ 4,535	$ 4,270
Other Variable Interest Entities
Other variable interest entities, purchase power agreement, plant capacity (in MW) | MW		11	61
MGE Power Elm Road [Member]
Significant Balance Sheet Accounts Related to Interests in Variable Interest Entities
Property, plant, and equipment, net		$ 173,464	$ 176,002
Construction work in progress		530	1,225
Affiliate receivables, current and noncurrent		0	0
Deferred income taxes		30,595	29,256
Long-term debt	[1]	56,806	59,416
Noncontrolling interest	[2]	97,519	97,635
Unamortized discount and debt issuance costs, net		500	600
MGE Power West Campus [Member]
Significant Balance Sheet Accounts Related to Interests in Variable Interest Entities
Property, plant, and equipment, net		81,273	80,282
Construction work in progress		478	610
Affiliate receivables, current and noncurrent		3,707	4,236
Deferred income taxes		14,222	13,795
Long-term debt	[1]	41,496	43,262
Noncontrolling interest	[2]	43,935	43,267
Unamortized discount and debt issuance costs, net		$ 100	$ 100

[1] (a) MGE Power Elm Road ' s long-term debt includes debt issuance costs of $ 0.5 million and $ 0.6 million as of December 31, 2018 and 2017 , respectively. The debt is secured by a collateral assignment of lease payments that MGE makes to MGE Power Elm Road for use of the Elm Road Units pursuant to the related long-term leases. MGE Power West Campus ' s long-term debt includes debt issuance cos ts of $ 0.1 million as of December 31, 2018 and 2017 . The debt is secured by a collateral assignment of lease payments that MGE makes to MGE Power West Campus for use of the cogeneration facility pursuant to the long-term lease. See Footnote 10 for further information on the long-term debt securities.

[2] MGE Power Elm Road and MGE Power West Campus are not su bsidiaries of MGE; however, they have been consolidated in the consolidated financial s tatements of MGE (see Footnote 3 ). MGE Power Elm Road and MGE Power West Campus are 100 % owned by MGE Power, and MGE Power is 100 % o wned by MGE Energy. MGE Energy's proportionate share of the equity and net income (through its wholly owned subsidiary MGE Power) of MGE Power Elm Road and MGE Power West Campus is classified within the MGE consolidated financial statements as noncontrolling interest . In 2017, there was a $14.8 million and $6.8 million one-time tax impact as a result of the Tax Act for MGE Power Elm Road and MGE Power West Campus, respectively. The Tax Act reduced the f ederal tax rate from 35% to 21%. See Footnote 12 for further information .