Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Financial information of reportable operating segments
Operating revenues	$ 1,698.7	$ 1,033.3	$ 4,077.8	$ 3,772.0
Other operation and maintenance	535.9	249.4	1,153.6	780.8
Depreciation and amortization	176.5	99.8	382.6	295.2
Operating income (loss)	345.7	246.1	870.3	868.6
Equity in earnings of transmission affiliate	40.0	18.0	70.4	52.8
Interest expense	103.8	60.4	225.6	181.7
Transmission Affiliate
Financial information of reportable operating segments
Equity in earnings of transmission affiliate	40.0	18.0	70.4	52.8
Eliminations and Reconciling Items
Financial information of reportable operating segments
Operating revenues	(103.6)	(98.7)	(307.1)	(294.3)
Other operation and maintenance	(103.6)	(97.4)	(306.6)	(290.5)
Depreciation and amortization	0.0	(0.1)	(0.1)	0.0
Operating income (loss)	0.0	0.0	0.0	0.0
Equity in earnings of transmission affiliate	0.1	0.0	0.2	0.0
Interest expense	(2.4)	(0.2)	(2.4)	(0.5)
Regulated Operation
Financial information of reportable operating segments
Operating revenues	1,663.7	1,017.7	4,020.5	3,729.6
Other operation and maintenance	626.0	337.4	1,363.4	1,048.6
Depreciation and amortization	153.6	82.8	325.3	244.1
Operating income (loss)	256.4	158.4	673.3	606.3
Equity in earnings of transmission affiliate	40.2	18.0	70.6	52.8
Interest expense	58.8	32.1	122.9	96.8
Regulated Operation | Wisconsin
Financial information of reportable operating segments
Operating revenues	1,442.3	1,017.7	3,799.1	3,729.6
Other operation and maintenance	501.9	337.4	1,239.3	1,048.6
Depreciation and amortization	117.5	82.8	289.2	244.1
Operating income (loss)	256.6	158.4	673.5	606.3
Equity in earnings of transmission affiliate	0.2	0.0	0.2	0.0
Interest expense	46.5	32.1	110.6	96.8
Regulated Operation | Illinois
Financial information of reportable operating segments
Operating revenues	171.8	0.0	171.8	0.0
Other operation and maintenance	101.0	0.0	101.0	0.0
Depreciation and amortization	31.2	0.0	31.2	0.0
Operating income (loss)	3.5	0.0	3.5	0.0
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Interest expense	9.7	0.0	9.7	0.0
Regulated Operation | Other States
Financial information of reportable operating segments
Operating revenues	49.6	0.0	49.6	0.0
Other operation and maintenance	23.1	0.0	23.1	0.0
Depreciation and amortization	4.9	0.0	4.9	0.0
Operating income (loss)	(3.7)	0.0	(3.7)	0.0
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Interest expense	2.6	0.0	2.6	0.0
Regulated Operation | Transmission Affiliate
Financial information of reportable operating segments
Operating revenues	0.0	0.0	0.0	0.0
Other operation and maintenance	0.0	0.0	0.0	0.0
Depreciation and amortization	0.0	0.0	0.0	0.0
Operating income (loss)	0.0	0.0	0.0	0.0
Equity in earnings of transmission affiliate	40.0	18.0	70.4	52.8
Interest expense	0.0	0.0	0.0	0.0
Unregulated Operation | We Power LLC
Financial information of reportable operating segments
Operating revenues	110.7	109.0	333.1	328.1
Other operation and maintenance	0.6	0.4	3.7	3.6
Depreciation and amortization	16.9	16.7	50.6	49.9
Operating income (loss)	93.2	92.0	278.9	274.6
Equity in earnings of transmission affiliate	0.0	0.0	0.0	0.0
Interest expense	15.8	16.1	47.6	48.6
Unregulated Operation | Corporate and Other
Financial information of reportable operating segments
Operating revenues	27.9	5.3	31.3	8.6
Other operation and maintenance	12.9	9.0	93.1	19.1
Depreciation and amortization	6.0	0.4	6.8	1.2
Operating income (loss)	(3.9)	(4.3)	(81.9)	(12.3)
Equity in earnings of transmission affiliate	(0.3)	0.0	(0.4)	0.0
Interest expense	$ 31.6	$ 12.4	$ 57.5	$ 36.8