Condensed Parent Company Financial Statements (Condensed Balance Sheet) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Cash and cash equivalents	$ 11.4	$ 159.3	$ 175.3	$ 346.9
Other	153.5	175.5
Total current assets	866.5	1,092.7
Investments in consolidated subsidiaries	249.6	238.5	229.2
Other	62.0	61.3
Total investments	300.8	289.2
Other assets	1,483.5	1,170.4
Total assets	9,687.9	9,282.9	9,036.0
Retained earnings	1,510.2	1,394.7
Accumulated other comprehensive loss	(0.8)	(1.4)
Shares in deferred compensation trust	8.3	7.6
Total common equity	3,013.0	2,893.6	2,772.6	2,823.5
Long-term debt, net	2,703.1	2,703.4
Total capitalization	5,923.0	5,842.8
Commercial paper	102.8	47.4
Other	127.5	160.7
Total current liabilities	855.0	866.7
Deferred income taxes	1,592.2	1,434.3
Other	259.6	208.9
Total long-term liabilities and deferred credits	2,909.9	2,573.4
Total capitalization and liabilities	9,687.9	9,282.9
Parent [Member]
Cash and cash equivalents		123.0	75.0	242.0
Notes receivable from affiliated companies	211	56
Income tax refunds receivable	11.0	16.0
Other	5.0
Total current assets	227.0	195.0
Investments in consolidated subsidiaries	3,129.0	2,966.0
Other	20.0	19.0
Total investments	3,149.0	2,985.0
Other assets	6.0	7.0
Total assets	3,382.0	3,187.0
Common stock and additional paid-in capital	1,512	1,508
Retained earnings	1,506.0	1,391.0
Accumulated other comprehensive loss	(1.0)	(1.0)
Shares in deferred compensation trust	(8.0)	(8.0)
Total common equity	3,009.0	2,890.0
Long-term debt, net	250.0	249.0
Total capitalization	3,259.0	3,139.0
Commercial paper	70.0
Other	14.0	6.0
Total current liabilities	84.0	6.0
Deferred income taxes	32.0	35.0
Other	7.0	7.0
Total long-term liabilities and deferred credits	39.0	42.0
Total capitalization and liabilities	$ 3,382.0	$ 3,187.0