Condensed Parent Company Financial Statements (Condensed Balance Sheet) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Current assets:
Other	$ 84.9	$ 89.4
Total current assets	1,011.2	994.3
Investments:
Other	57.5	62.0
Total investments	329.6	319.0
Other assets	1,445.1	1,634.2
Total assets	11,112.4	10,785.5	9,687.9
Capitalization:
Retained earnings	1,780.7	1,630.7
Accumulated other comprehensive loss	(0.2)	(0.8)
Shares in deferred compensation trust	(8.0)	(7.3)
Total common equity	3,281.4	3,134.9	3,013.0	2,893.6
Long-term debt, net	2,977.8	3,136.6
Total capitalization	6,461.0	6,478.4
Current liabilities:
Current maturities of long-term debt	358.5	1.5
Commercial paper	279.4	217.5
Other	133.1	176.0
Total current liabilities	1,433.3	1,020.0
Other long-term liabilities and deferred credits:
Non-current deferred tax liabilities	2,112.7	1,934.2
Other	273.9	262.5
Total long-term liabilities and deferred credits	3,218.1	3,287.1
Total capitalization and liabilities	11,112.4	10,785.5
Parent Company [Member]
Current assets:
Notes receivable from affiliated companies	72	77
Other	3.0	2.0
Total current assets	75.0	79.0
Investments:
Investments in consolidated subsidiaries	3,585	3,447
Other	14.0	20.0
Total investments	3,599.0	3,467.0
Other assets	6.0	5.0
Total assets	3,680.0	3,551.0
Capitalization:
Common stock and additional paid-in capital	1,509	1,512
Retained earnings	1,777.0	1,627.0
Accumulated other comprehensive loss	0	(1.0)
Shares in deferred compensation trust	(8.0)	(7.0)
Total common equity	3,278.0	3,131.0
Long-term debt, net	0	250.0
Total capitalization	3,278.0	3,381.0
Current liabilities:
Current maturities of long-term debt	250.0	0
Commercial paper	96.0	105.0
Other	10.0	21.0
Total current liabilities	356.0	126.0
Other long-term liabilities and deferred credits:
Non-current deferred tax liabilities	39.0	38.0
Other	7.0	6.0
Total long-term liabilities and deferred credits	46.0	44.0
Total capitalization and liabilities	$ 3,680.0	$ 3,551.0