Accounts Receivable (Schedule of Accounts Receivable) (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 188,341	$ 180,598
Allowance for doubtful accounts	(2,127)	(1,329)
Reserve for cash-basis customers	(3,549)	(3,903)
Total accounts receivable reserves	(5,676)	(5,232)
Accounts receivable, net	182,665	175,366
Trade Accounts Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	127,210	116,212
Unbilled Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 61,131	$ 64,386