Valuation and qualifying accounts (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Allowance for doubtful accounts
Valuation and qualifying accounts
Balance at the beginning of the year	$ 1,247	$ 1,200	$ 755
Additions charged to earnings	259	1,106	966
Changes to net	(906)	(1,059)	(521)
Balance at the end of the year	600	1,247	1,200
Reserves for inventory
Valuation and qualifying accounts
Balance at the beginning of the year	1,288	1,404	1,931
Additions charged to earnings	715	1,080	930
Changes to net	(804)	(1,196)	(1,457)
Balance at the end of the year	1,199	1,288	1,404
Valuation of deferred tax assets
Valuation and qualifying accounts
Balance at the beginning of the year	830	877	566
Additions charged to earnings			343
Changes to net	544	(47)	(32)
Balance at the end of the year	$ 1,374	$ 830	$ 877