Condensed Parent Company Financial Statements (Condensed Statements Of Income) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating revenues	$ 804.1	$ 843.1	$ 750.3	$ 952.8	$ 832.6	$ 866.6	$ 718.0	$ 859.6	$ 3,350.3	$ 3,276.8	$ 3,094.5
Operating expenses									2,806.7	2,742.9	2,574.8
Operating income (loss)	91.3	194.8	103.3	154.2	108.6	201.4	103.2	120.7	543.6	533.9	519.7
Interest expense and other:
Equity earnings from consolidated subsidiaries									(40.4)	(43.7)	(41.3)
Interest expense									180.6	172.8	156.7
Total interest expense and other									103.6	97.9	89.5
Income from continuing operations before income taxes									440.0	436.0	430.2
Income tax benefit									44.3	53.9	89.4
Net income									393.3	376.2	335.7
Parent Company [Member]
Operating revenues									2.0	2.0	2.0
Operating expenses									3.0	1.0	1.0
Operating income (loss)									(1.0)	1.0	1.0
Interest expense and other:
Equity earnings from consolidated subsidiaries									(387.0)	(362.0)	(322.0)
Interest expense									9.0	11.0	11.0
Interest income									(2)	(2)	(4)
Total interest expense and other									(380.0)	(353.0)	(315.0)
Income from continuing operations before income taxes									379.0	354.0	316.0
Income tax benefit									(4.0)	(4.0)	(4.0)
Net income									$ 383.0	$ 358.0	$ 320.0