Credit Facility (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	May 31, 2011	Dec. 31, 2011 Committed credit agreement	Dec. 31, 2010 Committed credit agreement	Dec. 31, 2011 Uncommitted credit agreements	Dec. 31, 2010 Uncommitted credit agreements
Line of Credit Facility [Line Items]
Credit Facilities, total facility		$ 1,000.0	$ 1,000.0	$ 650.0	$ 458.3	$ 455.2
Credit Facilities, amount outstanding			0	0	153.5	114.8
Credit Facilities, letters of credit			16.2	16.2	2.6	0.1
Credit Facilities, total available			983.8	633.8	302.2	340.3
Credit Facilities, weighted-average interest rate					5.00%	5.00%
Credit Agreement, amendment date	May 31, 2011
Credit Agreement, original agreement date	July 18, 2008
Credit Agreement, expiration date	May 31, 2016
Credit Agreement, borrowing capacity prior to amendment		650.0
Credit Agreement, limit on letters of credit	$ 200.0
Credit Agreement, margin for base rate advances	0.40%
Credit Agreement, margin for Eurocurrency rate advances	1.40%
Credit Agreement, annual rate for fronting fees accrued on letters of credit	0.25%
Credit Agreement, facility fee annual rate	0.35%