Consolidated Income Statements - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Operating revenues	$ 3,496.7	$ 3,625.0	$ 3,711.7
Operating expenses
Cost of sales	1,190.7	1,262.1	1,286.4
Other operation and maintenance	1,053.1	1,502.4	1,352.0
Depreciation and amortization	384.4	348.1	331.6
Property and revenue taxes	108.3	109.9	109.6
Total operating expenses	2,736.5	3,222.5	3,079.6
Operating income	760.2	402.5	632.1
Other income, net	22.7	20.2	13.2
Interest expense	477.4	120.1	117.3
Other expense	(454.7)	(99.9)	(104.1)
Income before income taxes	305.5	302.6	528.0
Income tax expense (benefit)	(57.8)	(56.9)	191.2
Net income	363.3	359.5	336.8
Preferred stock dividend requirements	1.2	1.2	1.2
Net income attributed to common shareholder	$ 362.1	$ 358.3	$ 335.6