Significant Customers and Concentration of Credit Risk (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Significant Customers and Concentration of Credit Risk
Accounts receivable	$ 188,250,000	$ 221,343,000
Revenues | Customer concentration | First significant customer
Significant Customers and Concentration of Credit Risk
Revenues from a significant customer	175,100,000	203,300,000	115,000,000
Percentage of revenues from a significant customer	12.60%	13.20%	9.30%
Accounts receivable	12,500,000	21,900,000
Revenues | Customer concentration | Second significant customer
Significant Customers and Concentration of Credit Risk
Revenues from a significant customer	165,300,000	297,800,000	198,600,000
Percentage of revenues from a significant customer	11.80%	19.40%	16.00%
Accounts receivable	$ 3,900,000	$ 8,000,000