Summary of Significant Accounting Policies (Details 2)	12 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2011 item	Dec. 31, 2010 item
Total revenues | Customer concentration risk | One customer
Concentration of risk
Number of customers	1	1	1
Percentage of concentration risk	18.20%	21.20%	18.60%
Total revenues | Customer concentration risk | Ten largest customers
Concentration of risk
Number of customers	10
Percentage of concentration risk	70.60%	68.60%	62.70%
Consolidated accounts receivable | Credit concentration risk
Concentration of risk
Number of customers	2
Consolidated accounts receivable | Credit concentration risk | One customer
Concentration of risk
Number of customers	1	1
Percentage of concentration risk	11.90%	27.20%
Consolidated accounts receivable | Credit concentration risk | Second customer
Concentration of risk
Number of customers	1
Percentage of concentration risk	11.50%