Accounts receivable, net (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Allowance for Doubtful Accounts Receivable [Roll Forward]
Balance at beginning of year	$ 2,644	$ 2,152	$ 1,943
Provision for losses	(279)	1,081	918
Write-offs, net of recoveries	(800)	(589)	(709)
Balance at end of year	$ 1,565	$ 2,644	$ 2,152