Concentration of Credit Risk (Details) (Accounts receivable, USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013 customer
Customer group one
Concentration of credit risk
Number of customers considered for concentration risk (customer)	3
Concentration risk receivable, minimum	$ 3.0
Concentration risk (as a percent)	27.40%
Customer group two
Concentration of credit risk
Number of customers considered for concentration risk (customer)	8
Concentration risk receivable, minimum	1.0
Concentration risk (as a percent)	25.80%
Concentration risk receivable, high end of range	$ 3.0