Accounts Receivable (Schedule of Accounts Receivable) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 237,005	$ 222,584
Allowance for doubtful accounts	(1,178)	(1,154)
Reserve for cash-basis customers	(3,499)	(2,653)
Total accounts receivable reserves	(4,677)	(3,807)
Accounts receivable, net	232,328	218,777
Trade Accounts Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	153,760	143,533
Unbilled Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 83,245	$ 79,051