Condensed Parent Company Financial Statements (Condensed Statements Of Income) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues	$ 873.5	$ 928.6	$ 816.1	$ 916.3	$ 856.1	$ 906.9	$ 765.3	$ 853.9	$ 3,534.5	$ 3,382.2	$ 3,320.0
Operating expenses									2,840.1	2,711.0	2,765.9
Operating income (loss)	$ 121.4	$ 256.1	$ 151.2	$ 165.7	$ 134.0	$ 236.3	$ 153.7	$ 147.2	694.4	671.2	554.1
Other (income) and deductions:
Equity earnings from consolidated subsidiaries									(54.6)	(44.8)	(39.6)
Interest expense									247.0	215.6	196.2
Total other (income) and deductions									124.4	138.4	110.7
Income from continuing operations before income taxes									570.0	532.8	443.4
Income tax benefit									47.7	66.7	59.4
Net income									522.3	467.5	381.7
Parent Company [Member]
Revenues									0.0	0.0	1.0
Operating expenses									5.0	2.0	3.0
Operating income (loss)									(5.0)	(2.0)	(2.0)
Other (income) and deductions:
Equity earnings from consolidated subsidiaries									(523.0)	(457.0)	(374.0)
Interest expense									4.0	3.0	3.0
Other									2.0	0.0	(2.0)
Total other (income) and deductions									(517.0)	(454.0)	(373.0)
Income from continuing operations before income taxes									512.0	452.0	371.0
Income tax benefit									(1.0)	(6.0)	(1.0)
Net income									$ 513.0	$ 458.0	$ 372.0