Condensed Parent Company Financial Statements (Condensed Statements Of Income) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating revenues	$ 797.0	$ 924.6	$ 754.6	$ 843.8	$ 740.1	$ 898.9	$ 717.2	$ 897.4	$ 3,320.0	$ 3,253.6	$ 3,350.3
Operating expenses									2,783.0	2,676.6	2,806.7
Operating income (loss)	$ 99.9	$ 162.6	$ 128.6	$ 145.9	$ 79.2	$ 235.9	$ 109.0	$ 152.9	537.0	577.0	543.6
Interest expense and other:
Equity earnings from consolidated subsidiaries									(39.6)	(33.8)	(40.4)
Interest expense									196.2	187.1	180.6
Total interest expense and other									93.6	115.7	103.6
Income from continuing operations before income taxes									443.4	461.3	440.0
Income tax benefit									59.4	70.4	44.3
Net income									381.7	388.4	393.3
Parent Company [Member]
Operating revenues									1.0	2.0	2.0
Operating expenses									3.0	3.0	3.0
Operating income (loss)									(2.0)	(1.0)	(1.0)
Interest expense and other:
Equity earnings from consolidated subsidiaries									(374.0)	(379.0)	(388.0)
Interest expense									3.0	3.0	9.0
Interest income									(2.0)	(3.0)	(2.0)
Total interest expense and other									(373.0)	(379.0)	(381.0)
Income from continuing operations before income taxes									371.0	378.0	380.0
Income tax benefit									(1.0)	(1.0)	(3.0)
Net income									$ 372.0	$ 379.0	$ 383.0