Debt Obligations (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Credit Facility [Member]
Debt Instrument [Line Items]
Maturity date	2017-12-01
Revolving loans	$ 300
Capacity for the issuance of standby letters of credit	125
Maximum increase in borrowing capacity	150
Interest rate description	Loans under the Credit Facility bear interest at LIBOR plus an applicable margin ranging from 1.75% to 2.50%, with the applicable margin varying according to the company's total leverage ratio, or, at the election of the company, at a base rate plus 0.75% to 1.50%
Variable rate basis of loans under the credit facility	LIBOR
Basis spread on base rate, minimum (in hundredths)	0.75%
Basis spread on base rate, maximum (in hundredths)	1.50%
Unused capacity, commitment fee description	quarterly commitment fee for the unused portion of the Credit Facility, if any, at a rate ranging from 0.30% to 0.50%
Letters of credit issued	3.7
Amount available for borrowings	257
Credit Facility [Member] | Minimum [Member]
Debt Instrument [Line Items]
Basis spread on variable rate (in hundredths)	1.75%
Commitment fee for the unused portion of the credit facility (in hundredths)	0.30%
Credit Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Basis spread on variable rate (in hundredths)	2.50%
Commitment fee for the unused portion of the credit facility (in hundredths)	0.50%
Senior Secured Term Loan [Member]
Debt Instrument [Line Items]
Face amount	144.4	150.0
Maturity date	2017-12-01
Fair value of debt	144.4	150.0
Interest rate description	("LIBOR") plus 1.75% per annum or a base rate plus 0.75% per annum
Quarterly principal payment amount	1.9
Remaining principal amount	$ 114.4
Variable rate basis of loans under the credit facility	LIBOR
Basis spread on variable rate (in hundredths)	1.75%
Basis spread on base rate, minimum (in hundredths)	0.75%