Leases - Maturity analysis of operating lease liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Jan. 01, 2019
Leases [Abstract]
Within one year	$ 3,227
After one year but within two years	3,004
After two year but within three years	2,664
After three year but within four years	2,168
After four year but within five years	2,063
After 5 years	11,546
Total undiscounted cash flows	24,672
Discount on cash flows	(4,497)
Operating lease liabilities	$ 20,175	$ 20,203