Condensed Parent Company Financial Statements (Condensed Statements Of Income) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating revenues	$ 750.9	$ 887.6	$ 690.3	$ 765.7	$ 760.8	$ 870.9	$ 712.5	$ 877.2	$ 3,094.5	$ 3,221.4	$ 3,262.1
Operating expenses									2,574.8	2,708.1	2,702.1
Operating income	101.6	213.7	108.8	95.6	106.0	209.4	70.6	127.3	519.7	513.3	560.0
Interest expense and other:
Equity earnings from consolidated subsidiaries									(41.3)	(39.3)	(38.1)
Interest expense									156.7	158.3	162.8
Total interest expense and other									89.5	102.7	102.1
Income from continuing operations before income taxes									430.2	410.6	457.9
Income tax benefit									(89.4)	(69.2)	(147.7)
Net income									335.7	321.9	306.3
Parent Company [Member]
Operating revenues									2.0	4.0	1.0
Operating expenses									1.0	1.0	2.0
Operating income									1.0	3.0	(1.0)
Interest expense and other:
Equity earnings from consolidated subsidiaries									(322.0)	(304.0)	(292.0)
Interest expense									11.0	11.0	11.0
Interest income									(4)	(2)	(1)
Total interest expense and other									(315.0)	(295.0)	(282.0)
Income from continuing operations before income taxes									316.0	298.0	281.0
Income tax benefit									(4.0)	(5.0)	(6.0)
Net income									$ 320.0	$ 303.0	$ 287.0