Goodwill and intangible assets (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2012	Feb. 22, 2013	Oct. 31, 2013
Changes in goodwill
Balance at the beginning of the period		$ 42,403	$ 26,744
Balance at the end of the period	42,403	42,403	26,744
Endeka Group Inc
Changes in goodwill
Balance at the beginning of the period				5,776
Additions	5,776
Balance at the end of the period				5,776
AWG
Changes in goodwill
Balance at the beginning of the period					9,883
Additions	9,883
Balance at the end of the period					$ 9,883