Report of Business Segments (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating revenues	$ 513.7		$ 567.0		$ 1,003.7	$ 1,059.7
Cost of sales	178.7		208.7		391.3	419.2
Other operation and maintenance	119.8		117.2		238.8	229.9
Depreciation and amortization	84.3		80.9		166.7	159.7
Taxes other than income	20.9		22.5		47.2	46.6
OPERATING INCOME	110.0		137.7		159.7	204.3
Equity in earnings of unconsolidated affiliates	35.8		29.3		66.5	63.2
Other income (expense)	1.5		2.5		(3.0)	5.7
Interest expense	35.9		40.9		70.5	79.5
Income tax expense (benefit)	11.2		17.9		5.4	28.0
Net income (loss)	100.2	$ 47.1	110.7	$ 55.0	147.3	165.7
Investment in unconsolidated affiliates	1,171.3		1,153.6		1,171.3	1,153.6	$ 1,177.5
Total assets	10,886.2		10,543.6		10,886.2	10,543.6	$ 10,748.6
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	513.7		567.0		1,003.7	1,059.7
Cost of sales	178.7		208.7		391.3	419.2
Other operation and maintenance	120.0		117.5		240.3	231.1
Depreciation and amortization	84.3		80.9		166.7	159.7
Taxes other than income	20.1		21.6		44.5	44.3
OPERATING INCOME	110.6		138.3		160.9	205.4
Equity in earnings of unconsolidated affiliates	0.0		0.0		0.0	0.0
Other income (expense)	1.5		4.0		4.1	8.5
Interest expense	33.3		39.2		65.7	76.5
Income tax expense (benefit)	4.3		11.1		5.2	14.1
Net income (loss)	74.5		92.0		94.1	123.3
Investment in unconsolidated affiliates	0.0		0.0		0.0	0.0
Total assets	9,733.8		9,447.8		9,733.8	9,447.8
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		0.3		0.7	0.6
Depreciation and amortization	0.0		0.0		0.0	0.0
Taxes other than income	0.1		0.2		0.3	0.4
OPERATING INCOME	(0.4)		(0.5)		(1.0)	(1.0)
Equity in earnings of unconsolidated affiliates	35.8		29.3		66.5	63.2
Other income (expense)	(0.4)		0.0		(7.8)	0.0
Interest expense	0.0		0.0		0.0	0.0
Income tax expense (benefit)	8.1		6.7		9.5	16.5
Net income (loss)	26.9		22.1		48.2	45.7
Investment in unconsolidated affiliates	1,157.2		1,144.6		1,157.2	1,144.6
Total assets	1,160.0		1,147.9		1,160.0	1,147.9
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	(0.5)		(0.6)		(2.2)	(1.8)
Depreciation and amortization	0.0		0.0		0.0	0.0
Taxes other than income	0.7		0.7		2.4	1.9
OPERATING INCOME	(0.2)		(0.1)		(0.2)	(0.1)
Equity in earnings of unconsolidated affiliates	0.0		0.0		0.0	0.0
Other income (expense)	1.0		(0.6)		1.7	(1.3)
Interest expense	3.2		2.6		5.8	4.5
Income tax expense (benefit)	(1.2)		0.1		(9.3)	(2.6)
Net income (loss)	(1.2)		(3.4)		5.0	(3.3)
Investment in unconsolidated affiliates	14.1		9.0		14.1	9.0
Total assets	104.5		95.6		104.5	95.6
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	0.0
Other income (expense)	(0.6)		(0.9)		(1.0)	(1.5)
Interest expense	(0.6)		(0.9)		(1.0)	(1.5)
Income tax expense (benefit)	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	$ (112.1)		$ (147.7)		$ (112.1)	$ (147.7)