Leases - Maturity analysis of operating lease liabilities (Details) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019
Leases [Abstract]
Within one year	$ 3,300
After one year but within two years	3,057
After two years but within three years	2,626
After three years but within four years	2,291
After four years but within five years	2,174
After 5 years	10,562
Total undiscounted cash flows	24,010
Discount on cash flows	(4,196)
Operating lease liabilities	$ 19,814	$ 20,200