Accounts Receivable (Schedule of Accounts Receivable) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 211,707	$ 215,491
Allowance for doubtful accounts	(1,706)	(1,627)
Reserve for cash-basis customers	(3,685)	(2,928)
Total accounts receivable reserves	(5,391)	(4,555)
Accounts receivable, net	206,316	210,936
Trade Accounts Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	143,804	142,166
Unbilled Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 67,903	$ 73,325