Credit Facility (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 12, 2013	Dec. 31, 2013 Committed credit agreement	Dec. 31, 2012 Committed credit agreement	Dec. 31, 2013 Uncommitted credit facility	Dec. 31, 2012 Uncommitted credit facility
Line of Credit Facility [Line Items]
Credit Facilities, total facility	$ 1,000.0		$ 1,000.0	$ 1,000.0	$ 700.2	$ 317.2
Credit Facilities, amount outstanding			0	0	179.1	172.1
Credit Facilities, letters of credit			14.3	15.1	4.2	3.3
Credit Facilities, total available			985.7	984.9	516.9	141.8
Line of Credit Facility Weighted Average Interest Rate					4.00%	4.00%
Line of Credit Facility Amendment Date		12/12/2013
Credit Agreement, expiration date	Dec. 12, 2018
Line of Credit Facility Commitment Increase	250.0
Credit Agreement, limit on letters of credit	$ 200.0
Credit Agreement, margin for base rate advances	0.275%
Credit Agreement, margin for Eurocurrency rate advances	1.275%
Credit Agreement, annual rate for fronting fees accrued on letters of credit	0.25%
Credit Agreement, facility fee annual rate	0.225%