Condensed Parent Company Financial Statements (Condensed Statements Of Income) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating expenses									$ 2,870.0	$ 2,840.1	$ 2,711.0
Operating income (loss)	$ 160.9	$ 290.2	$ 149.8	$ 176.8	$ 121.4	$ 256.1	$ 151.2	$ 165.7	777.7	694.4	671.2
Other (income) and deductions:
Equity earnings from consolidated subsidiaries									(53.0)	(54.6)	(44.8)
Interest expense									272.9	247.0	215.6
Total other (income) and deductions									141.6	124.4	138.4
Income from continuing operations before income taxes									636.1	570.0	532.8
Income tax benefit									68.7	47.7	66.7
Net income									567.4	522.3	467.5
Parent Company [Member]
Operating expenses									2.0	5.0	2.0
Operating income (loss)									(2.0)	(5.0)	(2.0)
Other (income) and deductions:
Equity earnings from consolidated subsidiaries									(562.0)	(523.0)	(457.0)
Interest expense									9.0	4.0	3.0
Other									7.0	2.0	0.0
Total other (income) and deductions									(546.0)	(517.0)	(454.0)
Income from continuing operations before income taxes									544.0	512.0	452.0
Income tax benefit									(15.0)	(1.0)	(6.0)
Net income									$ 559.0	$ 513.0	$ 458.0