INCOME STATEMENTS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Operating revenues	$ 1,411.9	$ 1,498.5	$ 1,485.4
Operating expenses
Cost of sales	540.6	602.0	573.9
Other operation and maintenance	396.2	448.0	447.6
Depreciation and amortization	166.2	141.9	139.3
Property and revenue taxes	41.3	40.2	39.5
Total operating expenses	1,144.3	1,232.1	1,200.3
Operating income	267.6	266.4	285.1
Other income, net	39.6	37.6	23.7
Interest expense	63.4	53.9	54.2
Other expense	(23.8)	(16.3)	(30.5)
Income before income taxes	243.8	250.1	254.6
Income tax expense	59.1	77.3	99.7
Net income	$ 184.7	$ 172.8	$ 154.9