Condensed Parent Company Financial Statements (Condensed Balance Sheet) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Current assets:
Income tax refunds receivable	$ 18.2	$ 39.1
Other	43.3	37.7
Total current assets	994.3	870.6
Investments:
Other	62.0	61.9
Total investments	319.0	300.7
Other assets	1,634.2	1,483.3
Total assets	10,785.5	9,687.9	9,282.9
Capitalization:
Retained earnings	1,630.7	1,510.2
Accumulated other comprehensive loss	(0.8)	(0.8)
Shares in deferred compensation trust	(7.3)	(8.3)
Total common equity	3,134.9	3,013.0	2,893.6	2,772.6
Long-term debt, net	3,136.6	2,703.1
Total capitalization	6,478.4	5,923.0
Current liabilities:
Commercial paper	217.5	102.8
Other	113.5	107.0
Total current liabilities	1,020.0	855.2
Other Liabilities And Deferred Credits Noncurrent [Abstract]
Deferred income taxes	1,934.2	1,592.2
Other	262.5	259.4
Total long-term liabilities and deferred credits	3,287.1	2,909.7
Total capitalization and liabilities	10,785.5	9,687.9
Parent Company [Member]
Current assets:
Notes receivable from affiliated companies	77	211
Income tax refunds receivable	0	11.0
Other	2.0	5.0
Total current assets	79.0	227.0
Investments:
Investments in consolidated subsidiaries	3,447	3,129
Other	20.0	20.0
Total investments	3,467.0	3,149.0
Other assets	5.0	6.0
Total assets	3,551.0	3,382.0
Capitalization:
Common stock and additional paid-in capital	1,512	1,512
Retained earnings	1,627.0	1,506.0
Accumulated other comprehensive loss	(1.0)	(1.0)
Shares in deferred compensation trust	(7.0)	(8.0)
Total common equity	3,131.0	3,009.0
Long-term debt, net	250.0	250.0
Total capitalization	3,381.0	3,259.0
Current liabilities:
Commercial paper	105.0	70.0
Other	21.0	14.0
Total current liabilities	126.0	84.0
Other Liabilities And Deferred Credits Noncurrent [Abstract]
Deferred income taxes	38.0	32.0
Other	6.0	7.0
Total long-term liabilities and deferred credits	44.0	39.0
Total capitalization and liabilities	$ 3,551.0	$ 3,382.0