Condensed Parent Company Financial Statements (Condensed Statements Of Income) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating revenues	$ 740.1	$ 898.9	$ 717.2	$ 897.4	$ 804.1	$ 843.1	$ 750.3	$ 952.8	$ 3,253.6	$ 3,350.3	$ 3,276.8
Operating expenses									2,676.6	2,806.7	2,742.9
Operating income (loss)	$ 79.2	$ 235.9	$ 109.0	$ 152.9	$ 91.3	$ 194.8	$ 103.3	$ 154.2	577.0	543.6	533.9
Interest expense and other:
Equity earnings from consolidated subsidiaries									(33.8)	(40.4)	(43.7)
Interest expense									187.1	180.6	172.8
Total interest expense and other									115.7	103.6	97.9
Income from continuing operations before income taxes									461.3	440.0	436.0
Income tax benefit									70.4	44.3	53.9
Net income									388.4	393.3	376.2
Parent Company [Member]
Operating revenues									2.0	2.0	2.0
Operating expenses									3.0	3.0	1.0
Operating income (loss)									(1.0)	(1.0)	1.0
Interest expense and other:
Equity earnings from consolidated subsidiaries									(379.0)	(388.0)	(361.0)
Interest expense									3.0	9.0	11.0
Interest income									(3.0)	(2.0)	(2.0)
Total interest expense and other									(379.0)	(381.0)	(352.0)
Income from continuing operations before income taxes									378.0	380.0	353.0
Income tax benefit									(1.0)	(3.0)	(5.0)
Net income									$ 379.0	$ 383.0	$ 358.0