Leases - Lease cost and other lease information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Lease Costs
Operating lease costs	$ 584		$ 1,117
Variable lease costs	119		230
Short-term lease costs	8		23
Net lease cost	711	$ 700	1,370	$ 1,400
Cash paid for amounts included in the measurement of lease liabilities
Operating lease cash flows - Fixed payments	570		1,083
Operating lease cash flows - Liability reduction	490		953
Right of use assets obtained during the period in exchange for operating lease liabilities	$ 764		$ 764
Weighted average lease term (in years)	6 years 9 months 21 days		6 years 9 months 21 days
Weighted average discount rate (in percent)	3.04%		3.04%