Report of Business Segments (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating revenues	$ 755.4			$ 698.8			$ 1,759.1	$ 1,758.5
Cost of sales	234.0			244.4			625.3	663.6
Other operation and maintenance	129.8			123.3			368.6	353.2
Depreciation and amortization	94.1			81.1			260.8	240.8
Taxes other than income	23.2			22.7			70.4	69.3
OPERATING INCOME	274.3			227.3			434.0	431.6
Equity in earnings of unconsolidated affiliates	38.3			40.1			104.8	103.3
Other income (expense)	(1.3)			6.7			(4.3)	12.4
Interest expense	39.6			38.7			110.1	118.2
Income tax expense	20.8			30.3			26.2	58.3
Net income (loss)	250.9	$ 100.2	$ 47.1	205.1	$ 110.7	$ 55.0	398.2	370.8
Investment in unconsolidated affiliates	1,175.9			1,160.6			1,175.9	1,160.6	$ 1,177.5
Total assets	11,083.7			10,703.0			11,083.7	10,703.0	$ 10,748.6
Electric Utility [Member]
Segment Reporting Information [Line Items]
Operating revenues	755.4			698.8			1,759.1	1,758.5
Cost of sales	234.0			244.4			625.3	663.6
Other operation and maintenance	130.0			121.1			370.3	352.2
Depreciation and amortization	94.1			81.1			260.8	240.8
Taxes other than income	22.3			21.9			66.8	66.2
OPERATING INCOME	275.0			230.3			435.9	435.7
Equity in earnings of unconsolidated affiliates	0.0			0.0			0.0	0.0
Other income (expense)	(0.1)			10.2			4.0	18.7
Interest expense	38.0			37.8			103.7	114.3
Income tax expense	9.7			18.8			14.9	32.9
Net income (loss)	227.2			183.9			321.3	307.2
Investment in unconsolidated affiliates	0.0			0.0			0.0	0.0
Total assets	10,019.7			9,652.0			10,019.7	9,652.0
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	0.6			0.5			1.3	1.1
Depreciation and amortization	0.0			0.0			0.0	0.0
Taxes other than income	0.1			0.1			0.4	0.5
OPERATING INCOME	(0.7)			(0.6)			(1.7)	(1.6)
Equity in earnings of unconsolidated affiliates	38.3			40.1			104.8	103.3
Other income (expense)	(0.6)			(1.2)			(8.4)	(1.2)
Interest expense	0.0			0.0			0.0	0.0
Income tax expense	8.5			9.9			18.0	26.4
Net income (loss)	28.5			28.4			76.7	74.1
Investment in unconsolidated affiliates	1,159.1			1,150.3			1,159.1	1,150.3
Total assets	1,161.5			1,153.9			1,161.5	1,153.9
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	(0.8)			1.7			(3.0)	(0.1)
Depreciation and amortization	0.0			0.0			0.0	0.0
Taxes other than income	0.8			0.7			3.2	2.6
OPERATING INCOME	0.0			(2.4)			(0.2)	(2.5)
Equity in earnings of unconsolidated affiliates	0.0			0.0			0.0	0.0
Other income (expense)	0.6			(0.3)			2.3	(1.6)
Interest expense	2.8			2.9			8.6	7.4
Income tax expense	2.6			1.6			(6.7)	(1.0)
Net income (loss)	(4.8)			(7.2)			0.2	(10.5)
Investment in unconsolidated affiliates	16.8			10.3			16.8	10.3
Total assets	108.3			184.0			108.3	184.0
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)	(1.2)			(2.0)			(2.2)	(3.5)
Interest expense	(1.2)			(2.0)			(2.2)	(3.5)
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	$ (205.8)			$ (286.9)			$ (205.8)	$ (286.9)