Segments and Related Information (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating revenues	$ 682.6	$ 670.8	$ 1,253.3	$ 1,242.9
Depreciation and amortization	(94.7)	(85.3)	(185.0)	(170.5)
Interest charges	(145.3)	(132.9)	(211.9)	(184.1)
Income tax (expense) benefit	9.3	(17.1)	15.1	(28.8)
Net income (loss)	(7.0)	32.0	(16.6)	58.4
Assets	17,806.4		17,806.4		$ 13,570.0
Capital expenditures			247.0	302.4	609.4
Operating Segments [Member] \| Electric Utility Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues	682.6	670.8	1,253.3	1,242.9
Depreciation and amortization	(94.7)	(85.3)	(185.0)	(170.5)
Interest charges	(50.1)	(49.0)	(100.2)	(98.1)
Income tax (expense) benefit	(39.8)	(51.2)	(49.9)	(64.3)
Net income (loss)	68.4	88.3	84.5	117.3
Assets	11,531.1		11,531.1		11,444.2
Capital expenditures			247.0		609.4
Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0	0.0	0.0
Depreciation and amortization	0.0	0.0	0.0	0.0
Interest charges	(103.2)	(91.9)	(127.7)	(102.0)
Income tax (expense) benefit	49.1	34.1	65.0	35.5
Net income (loss)	(75.4)	(56.3)	(101.1)	(58.9)
Assets	6,674.7		6,674.7		2,461.3
Capital expenditures			0.0		0.0
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0	0.0	0.0
Depreciation and amortization	0.0	0.0	0.0	0.0
Interest charges	8.0	8.0	16.0	16.0
Income tax (expense) benefit	0.0	0.0	0.0	0.0
Net income (loss)	0.0	$ 0.0	0.0	$ 0.0
Assets	$ (399.4)		(399.4)		(335.5)
Capital expenditures			$ 0.0		$ 0.0

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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