Report of Business Segments (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Operating revenues	$ 472.5	$ 755.4	$ 513.7	$ 490.0	$ 511.8	$ 698.8	$ 567.0	$ 492.7	$ 2,231.6	$ 2,270.3	$ 2,261.1
Cost of sales									786.9	892.5	897.6
Other operation and maintenance									491.8	474.6	458.7
Depreciation and amortization									355.0	321.6	283.5
Taxes other than income									93.6	92.0	89.4
Operating income (loss)	70.3	274.3	110.0	49.7	58.0	227.3	137.7	66.6	504.3	489.6	531.9
Equity in earnings of unconsolidated affiliates									113.9	152.8	131.2
Other income (expense)									(6.9)	11.3	50.4
Interest expense									147.9	156.0	143.8
Income tax expense (benefit)									29.8	72.2	(49.3)	[1]
NET INCOME	35.4	$ 250.9	$ 100.2	$ 47.1	54.7	$ 205.1	$ 110.7	$ 55.0	433.6	425.5	619.0
Investment in unconsolidated affiliates	1,151.5				1,177.5				1,151.5	1,177.5	1,160.4
Total assets	11,024.3				10,748.6				11,024.3	10,748.6	10,412.7
Capital expenditures									635.5	573.6	824.1
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues									2,231.6	2,270.3	2,261.1
Cost of sales									786.9	892.5	897.6
Other operation and maintenance									492.5	473.8	469.8
Depreciation and amortization									355.0	321.6	280.9
Taxes other than income									89.5	88.2	84.8
Operating income (loss)									507.7	494.2	528.0
Equity in earnings of unconsolidated affiliates									0.0	0.0	0.0
Other income (expense)									3.1	25.6	57.7
Interest expense									140.5	151.8	138.4
Income tax expense (benefit)									20.1	40.0	141.8	[1]
NET INCOME									350.2	328.0	305.5
Investment in unconsolidated affiliates	0.0				0.0				0.0	0.0	0.0
Total assets	10,076.6				9,704.5				10,076.6	9,704.5	9,255.6
Capital expenditures									635.5	573.6	824.1
Natural Gas Midstream Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues									0.0	0.0	0.0
Cost of sales									0.0	0.0	0.0
Other operation and maintenance									2.8	1.4	(0.8)
Depreciation and amortization									0.0	0.0	0.0
Taxes other than income									0.4	0.6	1.0
Operating income (loss)									(3.2)	(2.0)	(0.2)
Equity in earnings of unconsolidated affiliates									113.9	152.8	131.2
Other income (expense)									(8.6)	(4.9)	(1.0)
Interest expense									0.0	0.0	0.0
Income tax expense (benefit)									20.7	37.1	(195.2)	[1]
NET INCOME									81.4	108.8	325.2
Investment in unconsolidated affiliates	1,132.9				1,166.6				1,132.9	1,166.6	1,151.9
Total assets	1,135.4				1,169.8				1,135.4	1,169.8	1,155.3
Capital expenditures									0.0	0.0	0.0
Other Operations [Member]
Segment Reporting Information [Line Items]
Operating revenues									0.0	0.0	0.0
Cost of sales									0.0	0.0	0.0
Other operation and maintenance									(3.5)	(0.6)	(10.3)
Depreciation and amortization									0.0	0.0	2.6
Taxes other than income									3.7	3.2	3.6
Operating income (loss)									(0.2)	(2.6)	4.1
Equity in earnings of unconsolidated affiliates									0.0	0.0	0.0
Other income (expense)									2.2	(3.4)	(5.4)
Interest expense									11.0	10.2	6.3
Income tax expense (benefit)									(11.0)	(4.9)	4.1	[1]
NET INCOME									2.0	(11.3)	(11.7)
Investment in unconsolidated affiliates	18.6				10.9				18.6	10.9	8.5
Total assets	107.0				184.8				107.0	184.8	109.1
Capital expenditures									0.0	0.0	0.0
Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues									0.0	0.0	0.0
Cost of sales									0.0	0.0	0.0
Other operation and maintenance									0.0	0.0	0.0
Depreciation and amortization									0.0	0.0	0.0
Taxes other than income									0.0	0.0	0.0
Operating income (loss)									0.0	0.0	0.0
Equity in earnings of unconsolidated affiliates									0.0	0.0	0.0
Other income (expense)									(3.6)	(6.0)	(0.9)
Interest expense									(3.6)	(6.0)	(0.9)
Income tax expense (benefit)									0.0	0.0	0.0	[1]
NET INCOME									0.0	0.0	0.0
Investment in unconsolidated affiliates	0.0				0.0				0.0	0.0	0.0
Total assets	$ (294.7)				$ (310.5)				(294.7)	(310.5)	(107.3)
Capital expenditures									$ 0.0	$ 0.0	0.0
Tax Benefit Due to Tax Law Change [Member]
Segment Reporting Information [Line Items]
Income tax expense (benefit)											245.2
Tax Expense Due to Tax Law Change [Member]
Segment Reporting Information [Line Items]
Income tax expense (benefit)											$ 10.5

[1] The Company recorded an income tax benefit of $245.2 million and income tax expense of $10.5 million during the fourth quarter of 2017 due to the Company remeasuring deferred taxes related to the natural gas midstream operations and other operations segments, respectively, as a result of the 2017 Tax Act.