Accounts Receivable Schedule of Accounts Receivable (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 241,918	$ 222,584
Allowance for doubtful accounts	(1,137)	(1,154)
Reserve for cash-basis customers	(3,495)	(2,653)
Accounts receivable reserve	(4,632)	(3,807)
Accounts receivable, net	237,286	218,777
Trade Accounts Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	163,220	143,533
Unbilled Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 78,698	$ 79,051