Concentration of Credit Risk (Details) (Accounts receivable, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 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)	28.80%
Customer group two
Concentration of credit risk
Number of customers considered for concentration risk (customer)	5
Concentration risk receivable, minimum	1.0
Concentration risk receivable, high end of range	$ 3.0
Concentration risk (as a percent)	14.00%