Condensed Parent Company Financial Statements (Condensed Statements Of Income) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating revenues	$ 856.1	$ 906.9	$ 765.3	$ 853.9	$ 797.0	$ 924.6	$ 754.6	$ 843.8	$ 3,382.2	$ 3,320.0	$ 3,253.6
Operating expenses									2,728.8	2,783.0	2,676.6
Operating income (loss)	$ 129.7	$ 231.5	$ 149.3	$ 142.9	$ 99.9	$ 162.6	$ 128.6	$ 145.9	653.4	537.0	577.0
Interest expense and other:
Equity earnings from consolidated subsidiaries									(44.8)	(39.6)	(33.8)
Interest expense									215.6	196.2	187.1
Total interest expense and other									120.6	93.6	115.7
Income from continuing operations before income taxes									532.8	443.4	461.3
Income tax benefit									66.7	59.4	70.4
Net income									467.5	381.7	388.4
Parent Company [Member]
Operating revenues									0.0	1.0	2.0
Operating expenses									2.0	3.0	3.0
Operating income (loss)									(2.0)	(2.0)	(1.0)
Interest expense and other:
Equity earnings from consolidated subsidiaries									(457.0)	(374.0)	(379.0)
Interest expense									3.0	3.0	3.0
Interest income									0.0	(2.0)	(3.0)
Total interest expense and other									(454.0)	(373.0)	(379.0)
Income from continuing operations before income taxes									452.0	371.0	378.0
Income tax benefit									(6.0)	(1.0)	(1.0)
Net income									$ 458.0	$ 372.0	$ 379.0