CONDENSED CONSOLIDATED INCOME STATEMENTS - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Operating revenues	$ 379.2	$ 380.7	$ 1,122.9	$ 1,111.9
Operating expenses
Cost of sales	132.5	125.1	436.9	420.5
Other operation and maintenance	114.6	102.4	322.7	318.4
Depreciation and amortization	35.7	35.1	105.2	103.9
Property and revenue taxes	10.0	9.9	30.1	29.8
Total operating expenses	292.8	272.5	894.9	872.6
Operating income	86.4	108.2	228.0	239.3
Other income, net	8.9	5.5	27.0	17.1
Interest expense	13.4	13.7	39.4	41.2
Other expense	(4.5)	(8.2)	(12.4)	(24.1)
Income before income taxes	81.9	100.0	215.6	215.2
Income tax expense	24.9	39.1	66.1	84.3
Net income	$ 57.0	$ 60.9	$ 149.5	$ 130.9