Accounts Receivable (Schedule of Accounts Receivable) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 215,491	$ 180,598
Allowance for doubtful accounts	(1,627)	(1,329)
Reserve for cash-basis customers	(2,928)	(3,903)
Total accounts receivable reserves	(4,555)	(5,232)
Accounts receivable, net	210,936	175,366
Trade Accounts Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	142,166	116,212
Unbilled Revenues [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 73,325	$ 64,386