Financing Arrangements (Revolving Credit Facility and Letters of Credit) (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Oct. 31, 2011
Debt Instrument [Line Items]
Maximum amount available for borrowing under the revolving credit facility			$ 600,000,000
Line of credit facility, maturity date	Oct. 24, 2016
Additional borrowing capacity			200,000,000
Revolving credit facility outstanding balance	100,000,000
Revolving credit facility, remaining capacity	441,300,000	440,600,000
Line of credit default limit	50,000,000
Limit of uninsured judgment not stayed within 60 days	50,000,000
Limit of uninsured judgment not stayed, period (in days)	60 days
Standby Letters of Credit [Member]
Debt Instrument [Line Items]
Revolving credit facility, borrowing capacity			400,000,000
Letter of Credit [Member]
Debt Instrument [Line Items]
Maximum amount available for borrowing under the revolving credit facility			400,000,000
Outstanding letters of credit	58,700,000	59,400,000
Amounts drawn on letters of credit	0	0
Bridge Loan [Member]
Debt Instrument [Line Items]
Maximum amount available for borrowing under the revolving credit facility			$ 50,000,000
Line of Credit [Member]
Debt Instrument [Line Items]
Debt instrument basis description	a) the base rate (which is a rate per annum equal to the greatest of (i) the federal funds rate plus one-half of one percent, (ii) Bank of America, N.A.’s “prime rate” and (iii) the Eurodollar Rate (as such term is defined in the credit facility) for a one-month interest period plus one percent) plus an applicable margin ranging from 45 to 105 basis points or (b) the Eurodollar Rate plus an applicable margin ranging from 145 to 205 basis points
Line of Credit [Member] | Federal Funds [Member] | Variable Base Rate A [Member]
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate (as a percentage)	0.50%
Line of Credit [Member] | Eurodollar Rate Plus 45 to 105 Basis Points [Member] | Variable Base Rate A [Member]
Debt Instrument [Line Items]
Debt instrument, basis spread (as a percentage)	1.00%
Interest period on credit facility (in months)	1 month
Line of Credit [Member] | Eurodollar Rate Plus 45 to 105 Basis Points [Member] | Variable Base Rate A [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate (as a percentage)	0.45%
Line of Credit [Member] | Eurodollar Rate Plus 45 to 105 Basis Points [Member] | Variable Base Rate A [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate (as a percentage)	1.05%
Line of Credit [Member] | Eurodollar Rate Plus 145 to 205 Basis Points [Member] | Variable Base Rate B [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate (as a percentage)	1.45%
Line of Credit [Member] | Eurodollar Rate Plus 145 to 205 Basis Points [Member] | Variable Base Rate B [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate (as a percentage)	2.05%