Report of Business Segments (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating revenues	$ 511.8	$ 698.8	$ 567.0	$ 492.7	$ 501.9	$ 716.8	$ 586.4	$ 456.0	$ 2,270.3	$ 2,261.1		$ 2,259.2
Cost of sales									892.5	897.6		880.1
Other operation and maintenance									474.6	458.7		438.1
Depreciation and amortization									321.6	283.5		322.6
Taxes other than income									92.0	89.4		87.6
Operating income (loss)	58.0	227.3	137.7	66.6	85.7	249.1	147.3	49.8	489.6	531.9		530.8
Equity in earnings of unconsolidated affiliates									152.8	131.2		101.8
Other income (expense)									11.3	50.4		(4.2)
Interest expense									156.0	143.8		142.1
Income tax expense (benefit)									72.2	(49.3)	[1]	148.1
NET INCOME	54.7	$ 205.1	$ 110.7	$ 55.0	294.8	$ 183.4	$ 104.8	$ 36.0	425.5	619.0		338.2
Investment in unconsolidated affiliates	1,177.5				1,160.4				1,177.5	1,160.4		1,158.6
Total assets	10,748.6				10,412.7				10,748.6	10,412.7		9,939.6
Capital expenditures									573.6	824.1		660.1
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues									2,270.3	2,261.1		2,259.2
Cost of sales									892.5	897.6		880.1
Other operation and maintenance									473.8	469.8		451.2
Depreciation and amortization									321.6	280.9		316.4
Taxes other than income									88.2	84.8		84.0
Operating income (loss)									494.2	528.0		527.5
Equity in earnings of unconsolidated affiliates									0.0	0.0		0.0
Other income (expense)									25.6	57.7		9.1
Interest expense									151.8	138.4		138.1
Income tax expense (benefit)									40.0	141.8	[1]	114.4
NET INCOME									328.0	305.5		284.1
Investment in unconsolidated affiliates	0.0				0.0				0.0	0.0		0.0
Total assets	9,704.5				9,255.6				9,704.5	9,255.6		8,669.4
Capital expenditures									573.6	824.1		660.1
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									1.4	(0.8)		(0.1)
Depreciation and amortization									0.0	0.0		0.0
Taxes other than income									0.6	1.0		0.0
Operating income (loss)									(2.0)	(0.2)		0.1
Equity in earnings of unconsolidated affiliates									152.8	131.2		101.8
Other income (expense)									(4.9)	(1.0)		(7.7)
Interest expense									0.0	0.0		0.0
Income tax expense (benefit)									37.1	(195.2)	[1]	40.5
NET INCOME									108.8	325.2		53.7
Investment in unconsolidated affiliates	1,166.6				1,151.9				1,166.6	1,151.9		1,158.6
Total assets	1,169.8				1,155.3				1,169.8	1,155.3		1,521.6
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									(0.6)	(10.3)		(13.0)
Depreciation and amortization									0.0	2.6		6.2
Taxes other than income									3.2	3.6		3.6
Operating income (loss)									(2.6)	4.1		3.2
Equity in earnings of unconsolidated affiliates									0.0	0.0		0.0
Other income (expense)									(3.4)	(5.4)		(5.4)
Interest expense									10.2	6.3		4.2
Income tax expense (benefit)									(4.9)	4.1	[1]	(6.8)
NET INCOME									(11.3)	(11.7)		0.4
Investment in unconsolidated affiliates	10.9				8.5				10.9	8.5		0.0
Total assets	184.8				109.1				184.8	109.1		89.0
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)									(6.0)	(0.9)		(0.2)
Interest expense									(6.0)	(0.9)		(0.2)
Income tax expense (benefit)									0.0	0.0	[1]	0.0
NET INCOME									0.0	0.0		0.0
Investment in unconsolidated affiliates	0.0				0.0				0.0	0.0		0.0
Total assets	$ (310.5)				$ (107.3)				(310.5)	(107.3)		(340.4)
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. See Note 8 for further discussion of the effects of the 2017 Tax Act.