Debt (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Jun. 03, 2011
Line of Credit Facility [Line Items]
Credit Agreement Term	5 years
Debt instrument due date		June 3, 2016
Line of credit facility, amount outstanding	$ 539.0
Line of credit facility, interest rate during period	1.394%
Margin for LIBOR based loans, percentage	1.10%
Margin for base rate loans, percentage	0.10%
Line of credit facility commitment fee amount, percentage	0.15%
Covenant ratio debt EBITDA maximum numerator, through December 31, 2012	3.25
Covenant ratio debt EBITDA maximum denominator, through December 31, 2012	1
Covenant Ratio Debt EBITDA Maximum Numerator, Thereafter	3
Covenant Ratio Debt EBITDA Maximum Denominator Thereafter	1
Covenant ratio coverage maximum numerator	4
Covenant ratio coverage maximum denominator	1
Short Term Debt Letters Of Credit	20.0
Letters of Credit, Renewable Term	1 year
Unsecured Debt [Member]
Line of Credit Facility [Line Items]
Unsecured debt, credit agreement		1,250.0
Office Lease Obligations [Member]
Line of Credit Facility [Line Items]
Short Term Debt Letters Of Credit	$ 16.3
Federal Funds [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Credit facility interest rate, percentage	0.50%
LIBOR [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Credit facility interest rate, percentage	1.00%