Significant Customers and Concentration of Credit Risk (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Significant Customers and Concentration of Credit Risk
Accounts receivable from a significant customer	$ 221,343,000	$ 174,216,000
Revenues | Customer concentration | First significant customer
Significant Customers and Concentration of Credit Risk
Revenues from a significant customer	297,800,000	198,600,000	134,800,000
Percentage of revenues from a significant customer	19.40%	16.00%	15.70%
Accounts receivable from a significant customer	8,000,000	5,100,000
Revenues | Customer concentration | Second significant customer
Significant Customers and Concentration of Credit Risk
Revenues from a significant customer	203,300,000	115,000,000	81,600,000
Percentage of revenues from a significant customer	13.20%	9.30%	9.50%
Accounts receivable from a significant customer	$ 21,900,000	$ 13,100,000