CONDENSED INCOME STATEMENTS - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Operating revenues	$ 320.9	$ 350.1	$ 716.7	$ 743.7
Operating expenses
Cost of sales	113.3	128.6	293.6	304.4
Other operation and maintenance	96.9	105.4	201.3	208.1
Depreciation and amortization	40.8	33.6	81.1	69.5
Property and revenue taxes	10.1	10.1	20.3	20.1
Total operating expenses	261.1	277.7	596.3	602.1
Operating income	59.8	72.4	120.4	141.6
Other income, net	9.9	8.8	19.2	18.1
Interest expense	15.3	13.2	31.4	26.0
Other expense	(5.4)	(4.4)	(12.2)	(7.9)
Income before income taxes	54.4	68.0	108.2	133.7
Income tax expense	12.9	25.3	25.8	41.2
Net income	$ 41.5	$ 42.7	$ 82.4	$ 92.5