Leases - Components of lease expenses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Finance lease cost:
Amortization of assets	$ 1,038	$ 1,091	$ 3,115	$ 2,496
Interest on lease liabilities	475	505	1,446	1,204
Operating lease expense:
Operating lease cost	2,266	2,256	6,768	6,849
Variable lease cost	270	223	802	653
Sublease income	(49)	(46)	(144)	(139)
Total lease costs	$ 4,000	$ 4,029	$ 11,987	$ 11,063