Operations and Significant Accounting Policies (Details) - USD ($) $ in Millions		Jun. 30, 2017	Dec. 31, 2016
Inventories – Net [Abstract]
Fuel	[1]	$ 45.0	$ 43.9
Materials and Supplies		48.0	48.7
Raw Materials		2.8	2.9
Work in Progress		0.3	1.0
Finished Goods		8.3	8.6
Reserve for Obsolescence		(1.0)	(0.9)
Total Inventories – Net		103.4	104.2
Prepayments and Other Current Assets [Abstract]
Deferred Fuel Adjustment Clause		16.7	18.6
Restricted Cash		3.7	2.2
Other		18.4	19.5
Total Prepayments and Other Current Assets		38.8	40.3
Other Non-Current Assets [Abstract]
Contract Payment		28.4	29.6
Finance Receivable		11.5	11.5
Restricted Cash		8.6	8.6
Other		56.3	56.8
Total Other Non-Current Assets		104.8	106.5
Other Current Liabilites [Abstract]
PSAs		24.7	24.6
Other		45.4	49.1
Total Other Current Liabilities		70.1	73.7
Other Non-Current Liabilities [Abstract]
Asset Retirement Obligation		158.2	136.6
PSAs		101.5	113.8
Contingent Consideration	[2]	5.5	25.0
Other		43.9	47.3
Total Other Non-Current Liabilities		$ 309.1	$ 322.7

[1] Fuel consists primarily of coal inventory at Minnesota Power.

[2] Contingent Consideration relates to the estimated fair value of the earnings-based payment resulting from the U.S. Water Services acquisition. (See Note 5. Fair Value.)