Segments and Related Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Operating revenues	$ 670.8	$ 609.0	$ 1,242.9	$ 1,158.1
Depreciation and amortization	(85.3)	(83.5)	(170.5)	(163.3)
Interest (charges) income	(132.9)	(50.0)	(184.1)	(97.3)
Income tax (expense) benefit	(17.1)	(24.5)	(28.8)	(31.0)
Net income (loss)	32.0	44.4	58.4	63.3
Assets	11,010.3		11,010.3		$ 10,738.6
Capital expenditures			302.4	366.8	677.1
Operating Segments [Member] | Electric Utility Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues	670.8	609.0	1,242.9	1,158.1
Depreciation and amortization	(85.3)	(83.5)	(170.5)	(163.3)
Interest (charges) income	(49.0)	(47.9)	(98.1)	(93.2)
Income tax (expense) benefit	(51.2)	(25.6)	(64.3)	(33.5)
Net income (loss)	88.3	46.4	117.3	67.3
Assets	11,304.5		11,304.5		11,045.5
Capital expenditures			302.4		677.1
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) income	(91.9)	(10.1)	(102.0)	(20.1)
Income tax (expense) benefit	34.1	1.1	35.5	2.5
Net income (loss)	(56.3)	(2.0)	(58.9)	(4.0)
Assets	104.0		104.0		(51.1)
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) income	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	$ (398.2)		(398.2)		(255.8)
Capital expenditures			$ 0.0		$ 0.0