Fair Value of Financial Instruments and Concentrations of Credit Risk (Details 1) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Summary of Allowance for losses on Accounts Receivables
Balance at beginning of year	$ 5,839	$ 5,969	$ 4,259
Provision for uncollectible accounts	2,400	478	2,301
Write-offs, net of recoveries	(5,078)	(608)	(591)
Balance at end of year	$ 3,161	$ 5,839	$ 5,969