Report of Business Segments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Operating revenues	$ 698.8	$ 716.8	$ 1,758.5	$ 1,759.2
Cost of sales	244.4	255.7	663.6	696.5
Other operation and maintenance	123.3	111.9	353.2	340.9
Depreciation and amortization	81.1	76.9	240.8	207.2
Taxes other than income	22.7	23.2	69.3	68.4
OPERATING INCOME	227.3	249.1	431.6	446.2
Equity in earnings of unconsolidated affiliates	40.1	33.6	103.3	98.6
Other income (expense)	6.7	19.0	12.4	36.4
Interest expense	38.7	35.9	118.2	108.0
Income tax expense	30.3	82.4	58.3	149.0
Net income (loss)	205.1	183.4	370.8	324.2
Investment in unconsolidated affiliates	1,160.6	1,158.1	1,160.6	1,158.1	$ 1,160.4
Total assets	10,703.0	10,463.7	10,703.0	10,463.7	$ 10,412.7
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	698.8	716.8	1,758.5	1,759.2
Cost of sales	244.4	255.7	663.6	696.5
Other operation and maintenance	121.1	115.0	352.2	348.8
Depreciation and amortization	81.1	76.2	240.8	204.6
Taxes other than income	21.9	21.7	66.2	64.2
OPERATING INCOME	230.3	248.2	435.7	445.1
Equity in earnings of unconsolidated affiliates	0.0	0.0	0.0	0.0
Other income (expense)	10.2	19.5	18.7	39.2
Interest expense	37.8	34.5	114.3	103.7
Income tax expense	18.8	71.7	32.9	116.7
Net income (loss)	183.9	161.5	307.2	263.9
Investment in unconsolidated affiliates	0.0	0.0	0.0	0.0
Total assets	9,652.0	9,230.9	9,652.0	9,230.9
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	0.5	(0.4)	1.1	(0.1)
Depreciation and amortization	0.0	0.0	0.0	0.0
Taxes other than income	0.1	0.5	0.5	1.0
OPERATING INCOME	(0.6)	(0.1)	(1.6)	(0.9)
Equity in earnings of unconsolidated affiliates	40.1	33.6	103.3	98.6
Other income (expense)	(1.2)	0.4	(1.2)	0.5
Interest expense	0.0	0.0	0.0	0.0
Income tax expense	9.9	12.6	26.4	38.6
Net income (loss)	28.4	21.3	74.1	59.6
Investment in unconsolidated affiliates	1,150.3	1,152.9	1,150.3	1,152.9
Total assets	1,153.9	1,503.9	1,153.9	1,503.9
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	1.7	(2.7)	(0.1)	(7.8)
Depreciation and amortization	0.0	0.7	0.0	2.6
Taxes other than income	0.7	1.0	2.6	3.2
OPERATING INCOME	(2.4)	1.0	(2.5)	2.0
Equity in earnings of unconsolidated affiliates	0.0	0.0	0.0	0.0
Other income (expense)	(0.3)	(0.7)	(1.6)	(3.0)
Interest expense	2.9	1.6	7.4	4.6
Income tax expense	1.6	(1.9)	(1.0)	(6.3)
Net income (loss)	(7.2)	0.6	(10.5)	0.7
Investment in unconsolidated affiliates	10.3	5.2	10.3	5.2
Total assets	184.0	87.1	184.0	87.1
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)	(2.0)	(0.2)	(3.5)	(0.3)
Interest expense	(2.0)	(0.2)	(3.5)	(0.3)
Income tax expense	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	$ (286.9)	$ (358.2)	$ (286.9)	$ (358.2)