Leases - Maturity analysis of operating lease liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jan. 01, 2019
Leases [Abstract]
Within one year	$ 3,186
After one year but within two years	2,923
After two year but within three years	2,533
After three year but within four years	2,124
After four year but within five years	2,066
After 5 years	11,028
Total undiscounted cash flows	23,860
Discount on cash flows	(4,351)
Operating lease liabilities	$ 19,509	$ 20,203