Debt (Senior Credit Facility) (Details) (USD $)	9 Months Ended						9 Months Ended			9 Months Ended					9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Interest Rate Swap [Member]	Dec. 31, 2012 Interest Rate Swap [Member]	Sep. 30, 2013 Letter of Credit [Member]	Sep. 30, 2013 Term Loan Facility [Member]	Sep. 30, 2013 Senior Credit Facility [Member]	Dec. 31, 2012 Senior Credit Facility [Member]	Sep. 30, 2013 Senior Credit Facility [Member] Subsequent Event [Member]	Sep. 30, 2013 Senior Credit Facility [Member] Scheduled Repayment, Next Four Quarters [Member]	Sep. 30, 2013 Revolving Credit Facility [Member]	Dec. 31, 2012 Revolving Credit Facility [Member]	Oct. 04, 2013 Revolving Credit Facility [Member] Subsequent Event [Member]	Sep. 30, 2013 Term Loan or Revolving Credit Facility [Member] Subsequent Event [Member]	Sep. 30, 2013 Swingline loan [Member]
Line of Credit Facility [Line Items]
Debt instrument, face amount							$ 400,000,000
Debt instrument, maturity date										Oct. 04, 2018
Maximum borrowing capacity						300,000,000						850,000,000		1,000,000,000		30,000,000
Revolving Credit Facility	196,000,000		0
Credit agreement maximum borrowing capacity increase in additional borrowings															400,000,000
Letters of credit outstanding								118,400,000	152,200,000
Line of credit, current borrowing capacity												535,600,000	697,800,000
Line of credit, commitment fee (as a percentage)								0.175%
Payments on long-term debt	15,000,000	475,000,000					15,000,000
Credit facilities, scheduled repayments due											10,000,000
Derivative, notional amount				$ 170,000,000	$ 275,000,000