Accounts Receivable (Schedule of Accounts Receivable) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 222,584	$ 215,491
Allowance for doubtful accounts	(1,154)	(1,627)
Reserve for cash-basis customers	(2,653)	(2,928)
Total accounts receivable reserves	(3,807)	(4,555)
Accounts receivable, net	218,777	210,936
Trade Accounts Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	143,533	142,166
Unbilled Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 79,051	$ 73,325