Report of Business Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020		Mar. 31, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Operating revenues	$ 431.3		$ 490.0
Cost of sales	135.0		212.6
Other operation and maintenance	120.0		119.0
Depreciation and amortization	94.4		82.4
Taxes other than income	25.6		26.3
OPERATING INCOME	56.3		49.7
Equity in earnings of unconsolidated affiliates (A)	(746.5)	[1]	30.7
Other income (expense)	2.1		(4.5)
Interest expense	38.3		34.6
Income tax expense (benefit)	(234.6)		(5.8)
Net income (loss)	(491.8)		47.1
Investment in unconsolidated affiliates	367.5		1,173.9	$ 1,151.5
Total assets	10,370.9		10,761.6	$ 11,024.3
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	431.3		490.0
Cost of sales	135.0		212.6
Other operation and maintenance	121.0		120.3
Depreciation and amortization	94.4		82.4
Taxes other than income	23.9		24.4
OPERATING INCOME	57.0		50.3
Equity in earnings of unconsolidated affiliates (A)	0.0	[1]	0.0
Other income (expense)	1.8		2.6
Interest expense	36.9		32.4
Income tax expense (benefit)	2.0		0.9
Net income (loss)	19.9		19.6
Investment in unconsolidated affiliates	0.0		0.0
Total assets	10,030.7		9,478.5
Natural Gas Midstream Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0		0.0
Cost of sales	0.0		0.0
Other operation and maintenance	0.6		0.4
Depreciation and amortization	0.0		0.0
Taxes other than income	0.1		0.2
OPERATING INCOME	(0.7)		(0.6)
Equity in earnings of unconsolidated affiliates (A)	(746.5)	[1]	30.7
Other income (expense)	0.0		(7.4)
Interest expense	0.0		0.0
Income tax expense (benefit)	(179.2)		1.4
Net income (loss)	(568.0)		21.3
Investment in unconsolidated affiliates	348.0		1,162.0
Total assets	353.4		1,173.0
Other Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0		0.0
Cost of sales	0.0		0.0
Other operation and maintenance	(1.6)		(1.7)
Depreciation and amortization	0.0		0.0
Taxes other than income	1.6		1.7
OPERATING INCOME	0.0		0.0
Equity in earnings of unconsolidated affiliates (A)	0.0	[1]	0.0
Other income (expense)	1.3		0.7
Interest expense	2.4		2.6
Income tax expense (benefit)	(57.4)		(8.1)
Net income (loss)	56.3		6.2
Investment in unconsolidated affiliates	19.5		11.9
Total assets	252.9		200.5
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0.0		0.0
Cost of sales	0.0		0.0
Other operation and maintenance	0.0		0.0
Depreciation and amortization	0.0		0.0
Taxes other than income	0.0		0.0
OPERATING INCOME	0.0		0.0
Equity in earnings of unconsolidated affiliates (A)	0.0	[1]	0.0
Other income (expense)	(1.0)		(0.4)
Interest expense	(1.0)		(0.4)
Income tax expense (benefit)	0.0		0.0
Net income (loss)	0.0		0.0
Investment in unconsolidated affiliates	0.0		0.0
Total assets	$ (266.1)		$ (90.4)

[1] At March 31, 2020, the Company recorded a $780.0 million impairment on its investment in Enable, as further discussed in Notes 4 and 5.