Condensed Parent Company Financial Statements (Condensed Statements Of Cash Flow) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Net cash flows from operating activities	$ 702.7	$ 984.9	$ 657.1
Net cash flows used for investing activities	(652.1)	(866.5)	(1,148.9)
Common stock dividends	188.1	174.6	165.5
Net change in short-term borrowings	55.4	(142.6)	103.9
Proceeds from issuance of long-term debt	0.4	500.0	800.2
Payments to retire long-term debt	1.3	307.8	377.9
Other	(8.1)	9.3	(21.8)
Net cash flows from financing activities	(198.5)	(134.4)	320.2
Net increase (decrease) in cash and cash equivalents	(147.9)	(16.0)	(171.6)
Cash and cash equivalents at beginning of period	159.3	175.3	346.9
Cash and cash equivalents at end of period	11.4	159.3	175.3
Interest, net of capitalized interest	157.6	165.5	142.4
Income taxes, net of refunds	(10.8)	(116.2)	(140.7)
Parent [Member]
Net cash flows from operating activities	193.0	110.0	70.0
Capital contributions to consolidated subsidiaries	(144.0)	(125.0)	(250.0)
Capital repayments from consolidated subsidiaries	101.0	183.0	206.0
Dividends from consolidated subsidiaries in excess of equity earnings			5
Net cash flows used for investing activities	(43.0)	58.0	(39.0)
Common stock dividends	(188.0)	(175.0)	(166.0)
Net change in notes receivable from affiliates	(155)	50	(36)
Net change in short-term borrowings	70.0
Proceeds from issuance of common stock	2	6	1
Proceeds from issuance of long-term debt			250.0
Payments to retire long-term debt			(241.0)
Other	(2.0)	(1.0)	(6.0)
Net cash flows from financing activities	(273.0)	(120.0)	(198.0)
Net increase (decrease) in cash and cash equivalents	(123.0)	48.0	(167.0)
Cash and cash equivalents at beginning of period	123.0	75.0	242.0
Cash and cash equivalents at end of period		123.0	75.0
Interest, net of capitalized interest	11.0	11.0	11.0
Income taxes, net of refunds	$ (6.0)	$ (11.0)	$ 8.0