Credit Agreement (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012
Credit Agreement [Abstract]
Senior credit facility borrowings	$ 250
Line of Credit Facility [Line Items]
Line of credit facility, maturity date	October 2016
Borrowings for the line of credit	0
Percentage of additional interest, minimum (in hundredths)	0.40%
Percentage of additional interest, maximum (in hundredths)	1.00%
Percentage of additional interest, actual (in hundredths)	0.50%
Commitment fee for unused capacity, minimum (in hundredths)	0.08%
Commitment fee for unused capacity, maximum (in hundredths)	0.20%
Line of credit unused capacity fee percentage (in hundredths)	0.10%
Letters of Credit [Member]
Line of Credit Facility [Line Items]
Borrowing capacity	250
Letters of credit outstanding	23.0
Swing Line Loans [Member]
Line of Credit Facility [Line Items]
Borrowing capacity	$ 20