Debt (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		3 Months Ended		3 Months Ended
	Mar. 31, 2012 Commercial paper	Dec. 31, 2011 Commercial paper	Mar. 31, 2012 Revolving Credit Agreement Y	Mar. 20, 2012 Revolving Credit Agreement	Mar. 31, 2012 Revolving Credit Agreement Minimum	Mar. 31, 2012 Revolving Credit Agreement Maximum	Mar. 31, 2012 Revolving Credit Agreement LIBOR	Mar. 31, 2012 Revolving Credit Agreement Federal Funds rate	Mar. 31, 2012 Revolving Credit Agreement Prime rate	Mar. 31, 2012 Revolving Credit Agreement One month eurodollar rate
Debt Instrument
Total debt		$ 50
Weighted average commercial paper outstanding	20.9
Weighted-average interest rates (as a percent)	0.47%
New credit and loan facility				$ 400
Term of credit agreement			5
Debt instrument base interest rate							LIBOR	Federal fund effective rate	Prime rate	One month Eurodollar rate
Debt Instrument, Variable Rate								0.50%		1.00%
Ratio of consolidated adjusted EBITDA to consolidated interest expense , numerator					0.04
Ratio of consolidated adjusted EBITDA to consolidated interest expense , denominator					0.01
Ratio of consolidated funded debt to consolidated adjusted EBITDA, numerator					0.03	0.0325
Ratio of consolidated funded debt to consolidated adjusted EBITDA, denominator					0.01	0.01