Report of Business Segments (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020		Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Operating revenues	$ 702.1			$ 755.4			$ 1,636.9		$ 1,759.1
Cost of sales	209.1			234.0			481.5		625.3
Other operation and maintenance	109.7			129.8			347.2		368.6
Depreciation and amortization	100.5			94.1			292.2		260.8
Taxes other than income	24.8			23.2			76.3		70.4
OPERATING INCOME	258.0			274.3			439.7		434.0
Equity in earnings of unconsolidated affiliates	15.8			38.3			(703.8)	[1]	104.8
Other income (expense)	(0.8)			(1.3)			3.3		(4.3)
Interest expense	39.8			39.6			118.9		110.1
Income tax expense	55.8			20.8			(151.2)		26.2
Net income (loss)	177.4	$ 85.9	$ (491.8)	250.9	$ 100.2	$ 47.1	(228.5)		398.2
Investment in unconsolidated affiliates	376.8			1,175.9			376.8		1,175.9	$ 1,151.5
Total assets	10,497.6			11,083.7			10,497.6		11,083.7	$ 11,024.3
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	702.1			755.4			1,636.9		1,759.1
Cost of sales	209.1			234.0			481.5		625.3
Other operation and maintenance	110.1			130.0			348.8		370.3
Depreciation and amortization	100.5			94.1			292.2		260.8
Taxes other than income	23.9			22.3			72.8		66.8
OPERATING INCOME	258.5			275.0			441.6		435.9
Equity in earnings of unconsolidated affiliates	0.0			0.0			0.0	[1]	0.0
Other income (expense)	0.6			(0.1)			2.9		4.0
Interest expense	39.5			38.0			115.7		103.7
Income tax expense	20.1			9.7			30.5		14.9
Net income (loss)	199.5			227.2			298.3		321.3
Investment in unconsolidated affiliates	0.0			0.0			0.0		0.0
Total assets	10,323.3			10,019.7			10,323.3		10,019.7
Natural Gas Midstream Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0			0.0			0.0		0.0
Cost of sales	0.0			0.0			0.0		0.0
Other operation and maintenance	0.3			0.6			1.2		1.3
Depreciation and amortization	0.0			0.0			0.0		0.0
Taxes other than income	0.2			0.1			0.4		0.4
OPERATING INCOME	(0.5)			(0.7)			(1.6)		(1.7)
Equity in earnings of unconsolidated affiliates	15.8			38.3			(703.8)	[1]	104.8
Other income (expense)	(0.7)			(0.6)			(0.7)		(8.4)
Interest expense	0.0			0.0			0.0		0.0
Income tax expense	4.5			8.5			(167.2)		18.0
Net income (loss)	10.1			28.5			(538.9)		76.7
Investment in unconsolidated affiliates	355.8			1,159.1			355.8		1,159.1
Total assets	358.7			1,161.5			358.7		1,161.5
Other Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0			0.0			0.0		0.0
Cost of sales	0.0			0.0			0.0		0.0
Other operation and maintenance	(0.7)			(0.8)			(2.8)		(3.0)
Depreciation and amortization	0.0			0.0			0.0		0.0
Taxes other than income	0.7			0.8			3.1		3.2
OPERATING INCOME	0.0			0.0			(0.3)		(0.2)
Equity in earnings of unconsolidated affiliates	0.0			0.0			0.0	[1]	0.0
Other income (expense)	(0.5)			0.6			2.5		2.3
Interest expense	0.5			2.8			4.6		8.6
Income tax expense	31.2			2.6			(14.5)		(6.7)
Net income (loss)	(32.2)			(4.8)			12.1		0.2
Investment in unconsolidated affiliates	21.0			16.8			21.0		16.8
Total assets	109.6			108.3			109.6		108.3
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0			0.0			0.0		0.0
Cost of sales	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
Taxes other than income	0.0			0.0			0.0		0.0
OPERATING INCOME	0.0			0.0			0.0		0.0
Equity in earnings of unconsolidated affiliates	0.0			0.0			0.0	[1]	0.0
Other income (expense)	(0.2)			(1.2)			(1.4)		(2.2)
Interest expense	(0.2)			(1.2)			(1.4)		(2.2)
Income tax expense	0.0			0.0			0.0		0.0
Net income (loss)	0.0			0.0			0.0		0.0
Investment in unconsolidated affiliates	0.0			0.0			0.0		0.0
Total assets	$ (294.0)			$ (205.8)			$ (294.0)		$ (205.8)

[1] In March 2020, the Company recorded a $780.0 million impairment on its investment in Enable, as further discussed in Notes 4 and 5.