CONDENSED CONSOLIDATED INCOME STATEMENTS - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Operating revenues	$ 856.2	$ 855.4	$ 1,797.7	$ 1,827.4
Operating expenses
Cost of sales	270.9	273.9	627.9	622.5
Other operation and maintenance	366.6	324.4	702.2	651.0
Depreciation and amortization	86.9	82.7	172.2	164.8
Property and revenue taxes	27.3	28.3	54.5	56.7
Total operating expenses	751.7	709.3	1,556.8	1,495.0
Operating income	104.5	146.1	240.9	332.4
Other income, net	15.3	1.1	11.1	4.3
Interest expense	29.2	29.1	58.9	58.7
Other expense	(13.9)	(28.0)	(47.8)	(54.4)
Income before income taxes	90.6	118.1	193.1	278.0
Income tax (benefit) expense	(2.5)	42.5	(6.1)	100.3
Net income	93.1	75.6	199.2	177.7
Preferred stock dividend requirements	0.3	0.3	0.6	0.6
Net income attributed to common shareholder	$ 92.8	$ 75.3	$ 198.6	$ 177.1