Accounts Receivable (Schedule of Accounts Receivable) (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 183,174	$ 180,598
Allowance for doubtful accounts	(1,382)	(1,329)
Reserve for cash-basis customers	(3,532)	(3,903)
Total accounts receivable reserves	(4,914)	(5,232)
Accounts receivable, net	178,260	175,366
Trade Accounts Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	124,235	116,212
Unbilled Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 58,939	$ 64,386