Concentration of Credit Risk (Details) (Accounts receivable, USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013 customer
Customer group one
Concentration of credit risk
Number of customers considered for concentration risk (customer)	5
Concentration risk receivable, minimum	$ 3.0
Concentration risk (as a percent)	41.90%
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)	17.00%
Concentration risk receivable, high end of range	$ 3.0