Common Equity (Details) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
	Dec. 20, 2013	Dec. 31, 2015 $ / shares	Sep. 30, 2015 $ / shares	Jun. 30, 2015 $ / shares	Mar. 31, 2015 $ / shares	Dec. 31, 2014 $ / shares	Sep. 30, 2014 $ / shares	Jun. 30, 2014 $ / shares	Mar. 31, 2014 $ / shares	Dec. 31, 2015 USD ($) $ / shares	Dec. 31, 2014 USD ($) $ / shares	Dec. 31, 2013 USD ($) $ / shares
Class of Stock [Line Items]
Stock split declared December 20, 2013, ratio (3:2)	1.5
Common stock dividends declared										$ 40,043	$ 38,429	$ 37,107
Common stock dividends declared (in dollars per share) | $ / shares		$ 0.295	$ 0.295	$ 0.283	$ 0.283	$ 0.283	$ 0.283	$ 0.272	$ 0.272	$ 1.16	$ 1.11	$ 1.07
MGE [Member]
Class of Stock [Line Items]
Cash dividends paid to parent by MGE										$ 30,000	$ 26,500	$ 25,000