Report of Business Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating revenues	$ 490.0	$ 492.7
Cost of sales	212.6	210.5
Other operation and maintenance	119.0	112.7
Depreciation and amortization	82.4	78.8
Taxes other than income	26.3	24.1
OPERATING INCOME	49.7	66.6
Equity in earnings of unconsolidated affiliates	30.7	33.9
Other income (expense)	(4.5)	3.2
Interest expense	34.6	38.6
Income tax expense (benefit) (A)	(5.8)	10.1
Net income	47.1	55.0
Investment in unconsolidated affiliates	1,173.9	1,160.6	$ 1,177.5
Total assets	10,761.6	10,436.4	$ 10,748.6
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	490.0	492.7
Cost of sales	212.6	210.5
Other operation and maintenance	120.3	113.6
Depreciation and amortization	82.4	78.8
Taxes other than income	24.4	22.7
OPERATING INCOME	50.3	67.1
Equity in earnings of unconsolidated affiliates	0.0	0.0
Other income (expense)	2.6	4.5
Interest expense	32.4	37.3
Income tax expense (benefit) (A)	0.9	3.0
Net income	19.6	31.3
Investment in unconsolidated affiliates	0.0	0.0
Total assets	9,478.5	9,303.3
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	0.4	0.3
Depreciation and amortization	0.0	0.0
Taxes other than income	0.2	0.2
OPERATING INCOME	(0.6)	(0.5)
Equity in earnings of unconsolidated affiliates	30.7	33.9
Other income (expense)	(7.4)	0.0
Interest expense	0.0	0.0
Income tax expense (benefit) (A)	1.4	9.8
Net income	21.3	23.6
Investment in unconsolidated affiliates	1,162.0	1,150.6
Total assets	1,173.0	1,159.0
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	(1.7)	(1.2)
Depreciation and amortization	0.0	0.0
Taxes other than income	1.7	1.2
OPERATING INCOME	0.0	0.0
Equity in earnings of unconsolidated affiliates	0.0	0.0
Other income (expense)	0.7	(0.7)
Interest expense	2.6	1.9
Income tax expense (benefit) (A)	(8.1)	(2.7)
Net income	6.2	0.1
Investment in unconsolidated affiliates	11.9	10.0
Total assets	200.5	98.3
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	0.0	0.0
Other income (expense)	(0.4)	(0.6)
Interest expense	(0.4)	(0.6)
Income tax expense (benefit) (A)	0.0	0.0
Net income	0.0	0.0
Investment in unconsolidated affiliates	0.0	0.0
Total assets	$ (90.4)	$ (124.2)