Employee benefit plans - Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Employee Benefit Plans
Service cost	$ 1,955	$ 1,457
Interest cost	22	46
Expected return on plan assets	(15)	(30)
Amortization of prior service cost	54	54
Amortization of actuarial losses	288	167
Net periodic benefit cost	$ 2,304	$ 1,694
Location of costs excluding the service component	Other income (expense), net	Other income (expense), net
Expected contributions	$ 5,300