Long-Term Debt (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended			9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 item	Sep. 30, 2012	Dec. 31, 2009 item	Sep. 30, 2013 Revolver	Sep. 30, 2012 Revolver	Sep. 30, 2013 Revolver	Sep. 30, 2012 Revolver	Jun. 12, 2013 Revolver	Sep. 30, 2013 Term Loan
Credit agreements
Number of credit agreements					2
Borrowing base										$ 500.0
Number of credit facilities			2
Frequency of interest payments			3 months								3 months
Average interest rates (as a percent)						2.74%	3.39%	3.04%	3.23%
Average outstanding balance						330.6	304.3	406.7	299.0
Total interest and commitment fees	6.2	4.8	20.7	15.2
Repayment of borrowings								$ 167.0