Condensed Parent Company Financial Statements (Condensed Statements of Cash Flow) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net cash flows from operating activities	$ 841.1	$ 702.7	$ 984.9
Cash flows from (used for) investing activities:
Other	(19.9)	(4.6)	(16.7)
Net cash flows from (used for) investing activities	(1,155.5)	(652.1)	(866.5)
Cash flows used for financing activities:
Common stock dividends	(199.3)	(188.1)	(174.6)
Net change in commercial paper	164.7	55.4	(142.6)
Other	(8.9)	(8.1)	9.3
Net cash flows from (used for) financing activities	324.2	(198.5)	(134.4)
Net increase (decrease) in cash and cash equivalents	9.8	(147.9)	(16.0)
Cash and cash equivalents at beginning of period	11.4	159.3	175.3
Cash and cash equivalents at end of period	21.2	11.4	159.3
Supplemental cash flows information:
Interest, net of capitalized interest	155.2	157.6	165.5
Income taxes, net of refunds	(20.3)	(10.8)	(116.2)
Parent Company [Member]
Net cash flows from operating activities	260.0	193.0	110.0
Cash flows from (used for) investing activities:
Capital contributions to consolidated subsidiaries	(230.0)	(144.0)	(125.0)
Captial repayments from consolidated subsidiaries	0	101.0	183.0
Net change in notes receivable from affiliates	134	0	0
Other	1.0	0	0
Net cash flows from (used for) investing activities	(95.0)	(43.0)	58.0
Cash flows used for financing activities:
Common stock dividends	(199.0)	(188.0)	(175.0)
Net change in borrowings with affiliates	0	(155)	50
Net change in commercial paper	35.0	70.0	0
Proceeds from issuance of common stock	1	2	6
Other	(2.0)	(2.0)	(1.0)
Net cash flows from (used for) financing activities	(165.0)	(273.0)	(120.0)
Net increase (decrease) in cash and cash equivalents	0	(123.0)	48.0
Cash and cash equivalents at beginning of period	0	123.0	75.0
Cash and cash equivalents at end of period	0	0	123.0
Supplemental cash flows information:
Interest, net of capitalized interest	11.0	11.0	11.0
Income taxes, net of refunds	$ (29.0)	$ (6.0)	$ (11.0)