Leases - Maturity analysis of operating lease liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019
Leases [Abstract]
Within one year	$ 3,273
After one year but within two years	3,036
After two year but within three years	2,786
After three year but within four years	2,283
After four year but within five years	2,058
After 5 years	12,061
Total undiscounted cash flows	25,497
Discount on cash flows	(4,674)
Operating lease liabilities	$ 20,823	$ 20,203