Segments and Related Information (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating revenues	$ 570.7	$ 572.1
Depreciation and amortization	(90.3)	(85.2)
Interest charges	(66.6)	(51.2)
Income tax (expense) benefit	5.8	(11.7)
Net income (loss)	(9.6)	26.4
Assets	17,797.5		$ 13,570.0
Capital expenditures	116.6	133.6	609.4
Operating Segments [Member] | Electric Utility Segment [Member]
Segment Reporting Information [Line Items]
Operating revenues	570.7	572.1
Depreciation and amortization	(90.3)	(85.2)
Interest charges	(50.1)	(49.1)
Income tax (expense) benefit	(10.1)	(13.1)
Net income (loss)	16.1	29.0
Assets	11,465.6		11,444.2
Capital expenditures	116.6		609.4
Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0
Depreciation and amortization	0.0	0.0
Interest charges	(24.5)	(10.1)
Income tax (expense) benefit	15.9	1.4
Net income (loss)	(25.7)	(2.6)
Assets	6,729.5		2,461.3
Capital expenditures	0.0		0.0
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0	0.0
Depreciation and amortization	0.0	0.0
Interest charges	8.0	8.0
Income tax (expense) benefit	0.0	0.0
Net income (loss)	0.0	$ 0.0
Assets	(397.6)		(335.5)
Capital expenditures	$ 0.0		$ 0.0