Goodwill and intangible assets (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Changes in intangible assets
Balance at the beginning of the period	$ 10,594	$ 9,846
Additions	15,069	1,851
Amortization expense	(2,250)	(1,103)	(1,655)
Balance at the end of the period	$ 23,413	$ 10,594	$ 9,846