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	$ 350.1	$ 341.6	$ 743.7	$ 731.2
Operating expenses
Cost of sales	128.6	130.5	304.4	295.4
Other operation and maintenance	105.4	108.8	208.1	216.0
Depreciation and amortization	33.6	34.4	69.5	68.8
Property and revenue taxes	10.1	9.8	20.1	19.9
Total operating expenses	277.7	283.5	602.1	600.1
Operating income	72.4	58.1	141.6	131.1
Other income, net	8.8	5.9	18.1	11.6
Interest expense	13.2	13.6	26.0	27.5
Other expense	(4.4)	(7.7)	(7.9)	(15.9)
Income before income taxes	68.0	50.4	133.7	115.2
Income tax expense	25.3	19.7	41.2	45.2
Net income	$ 42.7	$ 30.7	$ 92.5	$ 70.0