Accounts Receivable (Schedule of Accounts Receivable) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 242,010	$ 215,491
Allowance for doubtful accounts	(1,791)	(1,627)
Reserve for cash-basis customers	(3,987)	(2,928)
Total accounts receivable reserves	(5,778)	(4,555)
Accounts receivable, net	236,232	210,936
Trade Accounts Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	170,963	142,166
Unbilled Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 71,047	$ 73,325