Intangible assets, Additional Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	$ 7,719				$ 7,719				$ 7,719	$ 7,719
Accumulated amortization	(4,265)				(2,944)				(4,265)	(2,944)
Net carrying value	3,454				4,775				3,454	4,775
Amortization of intangible assets and other	316	335	335	335	368	397	391	367	1,321	1,522	866
Purchased Technology [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying value	350				350				350	350
Accumulated amortization	(350)				(288)				(350)	(288)
Net carrying value	0				62				0	62
Amortization of intangible assets and other									$ 62	$ 81	$ 83