Debt (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended		9 Months Ended		9 Months Ended
	Sep. 30, 2012 Commercial paper	Sep. 30, 2012 Commercial paper	Dec. 31, 2011 Commercial paper	Sep. 30, 2012 Revolving Credit Agreement	Mar. 20, 2012 Revolving Credit Agreement	Sep. 30, 2012 Revolving Credit Agreement Minimum	Sep. 30, 2012 Revolving Credit Agreement Maximum	Sep. 30, 2012 Revolving Credit Agreement LIBOR	Sep. 30, 2012 Revolving Credit Agreement Federal Funds rate	Sep. 30, 2012 Revolving Credit Agreement Prime rate	Sep. 30, 2012 Revolving Credit Agreement One month Eurodollar rate	Sep. 30, 2012 3.125% Senior notes due 2016	Sep. 30, 2012 8.205% Junior subordinated deferrable interest debentures due January 2027	Sep. 30, 2012 5.00% Senior notes due September 2020	Sep. 30, 2012 3.50% senior notes due September 2015	Sep. 30, 2012 6.25% Senior notes due September 2040	Sep. 30, 2012 7.375% Debt securities due December 2012
Debt Instrument
Total debt			$ 50
Weighted average commercial paper outstanding	15.0	41.0
Weighted-average interest rates (as a percent)	0.52%	0.51%
New credit and loan facility					$ 400
Term of credit agreement				5 years
Debt instrument base interest rate								LIBOR	Federal fund effective rate	Prime rate	One month Eurodollar rate
Debt Instrument, Variable Rate (as a percent)									0.50%		1.00%	3.125%	8.205%	5.00%	3.50%	6.25%	7.375%
Ratio of consolidated funded debt to consolidated adjusted EBITDA						0.0300	0.0325