Goodwill and intangible assets (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Changes in carrying amount of goodwill
Balance at the beginning of the period	$ 25,512	$ 25,512
Additions	1,232
Balance at the end of the period	26,744	25,512	25,512
Changes in carrying amount of intangible assets
Additions	1,851	250
Amortization	(1,103)	(1,655)	(2,491)
Intangible assets, net	10,594	9,846	11,251
Changes in carrying amount of goodwill and intangible assets
Balance at the beginning of the period	35,358	36,763
Additions	3,083	250
Amortization	(1,103)	(1,655)
Balance at the end of the period	$ 37,338	$ 35,358	$ 36,763