Goodwill and intangible assets - Intangible assets rollforward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Changes in intangible assets
Balance at beginning of year	$ 10,263	$ 13,783
Additions	12,640
Amortization expense	(3,751)	(3,520)
Balance at end of year	$ 19,152	$ 10,263