Segments and Related Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Operating revenues	$ 856.8	$ 781.4	$ 2,099.7	$ 1,939.5
Depreciation and amortization	(86.4)	(82.4)	(256.9)	(245.7)
Interest (charges) income	(67.6)	(51.0)	(251.7)	(148.3)
Income tax (expense) benefit	(82.7)	(78.6)	(111.5)	(109.6)
Net income (loss)	133.6	126.8	192.0	190.1
Assets	11,044.7		11,044.7		$ 10,738.6
Capital expenditures			435.3	520.9	677.1
Operating Segments [Member] | Electric Utility Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues	856.8	781.4	2,099.7	1,939.5
Depreciation and amortization	(86.4)	(82.4)	(256.9)	(245.7)
Interest (charges) income	(49.3)	(48.9)	(147.4)	(142.1)
Income tax (expense) benefit	(95.9)	(78.5)	(160.2)	(112.0)
Net income (loss)	161.1	129.1	278.4	196.4
Assets	11,337.1		11,337.1		11,045.5
Capital expenditures			435.3		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	(26.4)	(10.2)	(128.4)	(30.3)
Income tax (expense) benefit	13.2	(0.1)	48.7	2.4
Net income (loss)	(27.5)	(2.3)	(86.4)	(6.3)
Assets	114.5		114.5		(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	8.1	8.1	24.1	24.1
Income tax (expense) benefit	0.0	0.0	0.0	0.0
Net income (loss)	0.0	$ 0.0	0.0	$ 0.0
Assets	$ (406.9)		(406.9)		(255.8)
Capital expenditures			$ 0.0		$ 0.0