Credit Facility (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	May 31, 2011
Line of Credit Facility [Line Items]
Credit Facilities, total facility	$ 1,000.0
Credit Facilities, weighted-average interest rate		5.00%
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%
Covenant Modification Expiration Date	Aug. 15, 2013
Committed credit agreement
Line of Credit Facility [Line Items]
Credit Facilities, total facility	1,000.0	1,000.0
Credit Facilities, amount outstanding	0	0
Credit Facilities, letters of credit	15.1	16.2
Credit Facilities, total available	984.9	983.8
Uncommitted credit agreements
Line of Credit Facility [Line Items]
Credit Facilities, total facility	317.2	458.3
Credit Facilities, amount outstanding	172.1	153.5
Credit Facilities, letters of credit	3.3	2.6
Credit Facilities, total available	$ 141.8	302.2
Credit Facilities, weighted-average interest rate	4.00%	5.00%