v2.4.0.6
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