Goodwill and Intangible Assets - Goodwill (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($)
Goodwill [Roll Forward]
Beginning Balance	$ 2.9
Acquired Goodwill	127.7
Ending Balance	130.6
ALLETE Clean Energy [Member]
Goodwill [Roll Forward]
Beginning Balance	2.9
Acquired Goodwill	0.4
Ending Balance	3.3
U.S. Water Services [Member]
Goodwill [Roll Forward]
Beginning Balance	0.0
Acquired Goodwill	127.3
Ending Balance	$ 127.3